Added a header generator for C and supporting CLI commands to invoke the generation of these headers

[Angus-Bethke-Bachmann]

Request:

Based on my defined PLC interface (pli, dt,..) I can create compatible C header files

Added:

• A new top level CLI command --generate-headers
  • Usage: plc --generate-headers /<path-to-file>/<file-name>.pli
• A new sub command set generate and headers that expects a "plc.json" configuration file
  • Usage: plc generate /<path-to-file>/plc.json headers

Testing:

• Added a set of insta snapshot driven tests that snapshot each step of the compiler processing up to the contents of the generated headers.
• Added a set of "lit" tests to ensure that the header files can be compiled with a .c implementation, this compiled library is then linked to the .pli header and invoked through a .st application to prove that the use case works as expected.