Added support for processing library source files

[mkalkbrenner]

closes #112

[maxgerhardt]

Technically that neglects sources in the lib/ folder of the project (only .pio/libdeps is looked at) and the global library sources.

One could iterate over the paths in env["LIBSOURCE_DIRS"] as that includes all three:

https://github.com/platformio/platformio-core/blob/4850c1069c67d7074e838238fe1b0316cc7127b1/platformio/builder/main.py#L131C5-L135