Improve PlatformIO project handling

[maxgerhardt]

Just some pointers for later:

1. in the platformio.ini, instead of platform = ch32v (which expects an already installed ch32v platform), you can point to platform = https://github.com/Community-PIO-CH32V/platform-ch32v.git#18e295efcb1fee51f2b3855a9cccf886df251d88
   • this enables the project to be opened in PlatformIO/VSCode without having to previously open ch32v003fun in VSCode
   • this enables the project to be stable, pointing to a fixed commit of the platform (which contains a bunch of build logic, compiler version references etc.)
2. you can additionally add platform_packages = framework-ch32v003fun@ https://github.com/cnlohr/ch32v003fun.git#f06633d2fbd13c6d240f8c52041a1b0f52dbce90 to fixate the ch32v003fun framework version the project is built against. (Taking the latest commit, which might not be your current lcal version -- hopefully there are no breaking changes)
   • This would enable the project to have a reproducable build and enable easier collaboration (since everyone's on the same version)
3. Create a .github/workflows/build.yml file and copy-paste in the contents from https://docs.platformio.org/en/latest/integration/ci/github-actions.html and then change pio run to pio run -d firmware
   • this enables Github CI actions to automatically trigger a build of the firmware on a Ubuntu machine with PlatformIO
   • project gets the green checkmark in the github homepage