Add "smoke test" sketch compilation CI workflow for MuxTO firmware

On every push and pull request that affects relevant files, and periodically, compile the Nano Every's MuxTO firmware sketch.

The binary is saved as a workflow artifact to make it available for beta testers who might prefer flashing it directly
over setting up the toolchain needed to compile it locally.

Due to the standard output from Arduino CLI not providing the correct size data, a bespoke check is done for whether the
compiled binary will fit in the available 12 kB of memory. If not, the workflow run fails.

On creation or commit to a pull request, a report of the resulting change in memory usage of the sketch will be
commented to the PR thread.

.github/workflows/compile-muxto.yml

+name: Compile MuxTO
+
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/compile-muxto.yml"
+      - "firmwares/MuxTO/**"
+  pull_request:
+    paths:
+      - ".github/workflows/compile-muxto.yml"
+      - "firmwares/MuxTO/**"
+  schedule:
+    # Run every Tuesday at 8 AM UTC to catch breakage caused by changes to external resources (libraries, platforms).
+    - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
+env:
+  BINARY_FILENAME: MuxTO.ino.bin
+  BINARY_ARTIFACT_NAME: MuxTO
+
+jobs:
+  build:
+    name: Build firmware
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+
+      matrix:
+        board:
+          - fqbn: arduino:samd:muxto:float=default,config=enabled,clock=internal_usb,timer=timer_732Hz,bootloader=4kb,serial=two_uart,usb=cdc
+            platforms: |
+              # Install MattairTech_Arduino:samd via Boards Manager for the toolchain
+              - name: MattairTech_Arduino:samd
+                source-url: https://www.mattairtech.com/software/arduino/package_MattairTech_index.json
+                # This needs to match with the version of MattairTech_Arduino:samd the Arduino fork is based on in order to get the right tool versions
+                version: 1.6.17
+              # Install the platform with MuxTO support
+              - name: arduino:samd
+                source-url: https://github.com/arduino/ArduinoCore-samd.git
+                version: muxto
+
+    steps:
+      - name: Set environment variables
+        run: |
+          # See: https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#setting-an-environment-variable
+          echo "BINARY_OUTPUT_PATH=${{ runner.temp }}/output" >> "$GITHUB_ENV"
+          echo "SKETCHES_REPORTS_PATH=${{ runner.temp }}/sketches-reports" >> "$GITHUB_ENV"
+
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Compile firmware
+        uses: arduino/compile-sketches@v1
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          fqbn: ${{ matrix.board.fqbn }}
+          platforms: ${{ matrix.board.platforms }}
+          libraries: |
+            -
+          sketch-paths: |
+            - firmwares/MuxTO
+          cli-compile-flags: |
+            - --output-dir=${{ env.BINARY_OUTPUT_PATH }}
+          enable-deltas-report: true
+          sketches-report-path: ${{ env.SKETCHES_REPORTS_PATH }}
+
+      - name: Save firmware binary as workflow artifact
+        uses: actions/upload-artifact@v2
+        with:
+          if-no-files-found: error
+          path: ${{ env.BINARY_OUTPUT_PATH }}/${{ env.BINARY_FILENAME }}
+          name: ${{ env.BINARY_ARTIFACT_NAME }}
+
+      - name: Save sketches report as workflow artifact
+        uses: actions/upload-artifact@v2
+        with:
+          if-no-files-found: error
+          path: ${{ env.SKETCHES_REPORTS_PATH }}
+          name: sketches-reports
+
+  size:
+    name: Check firmware size
+    needs: build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download binary artifact
+        uses: actions/download-artifact@v2
+        with:
+          name: ${{ env.BINARY_ARTIFACT_NAME }}
+
+      # The normal size check done by Arduino CLI is not working correctly, so it's necessary to check the size directly
+      - name: Check firmware binary size
+        run: |
+          BINARY_SIZE="$(stat --printf="%s" "${{ github.workspace }}/${{ env.BINARY_FILENAME }}")"
+          MAX_BINARY_SIZE=$((12 * 1024))
+          echo "File size: ${BINARY_SIZE}/${MAX_BINARY_SIZE} B"
+          if [[ $BINARY_SIZE -gt $MAX_BINARY_SIZE ]]; then
+            echo "::error::Binary size of $BINARY_SIZE B exceeds the available memory ($MAX_BINARY_SIZE B)"
+            exit 1
+          fi

README.md

 # Arduino Core for ATMEGA4809 CPU
 
+[![Compile MuxTO status](https://github.com/arduino/ArduinoCore-megaavr/actions/workflows/compile-muxto.yml/badge.svg)](https://github.com/arduino/ArduinoCore-megaavr/actions/workflows/compile-muxto.yml)
+
 This repository contains the source code and configuration files of the Arduino Core
 for Microchip's ATMEGA4809 processor (used on the Arduino Uno WiFi Rev2 boards).