feat: Create pio.yml testing PlatformIO build (#6)

* Create pio.yml testing PlatformIO build

Runs `pio run` on all pushes to check for compilability.

* Expand Cache in pio.yml

Caches the whole ~/.platformio directory, as well as the build itself in .pio/

.github/workflows/pio.yml

@@ -0,0 +1,42 @@
+name: PlatformIO CI
+
+on:
+  push:
+    paths:
+      - '**.kicad_sch'
+      - '**.kicad_pcb'
+      - '**.kicad_pro'
+      - '.github/**.yml'
+  pull_request:
+    paths:
+      - '**.kicad_sch'
+      - '**.kicad_pcb'
+      - '**.kicad_pro'
+      - '.github/**.yml'
+  merge_group:
+
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cache/pip
+            ~/.platformio/
+            .pio/
+          key: ${{ runner.os }}-pio
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+      - name: Install PlatformIO Core
+        run: pip install --upgrade platformio
+
+      - name: Build PlatformIO Project
+        run: pio run