Add very basic support for FlexPRET platform

[magnmaeh]

Current status: Can run with the following output. Seems to hang when it should print hello world - could be lots of reasons for that.

magnus@magnus-XPS-15-9570:~/dev/lingua-franca/test/uC/main$ ./bin/fp-lf-uc 
[0]: Assembling environment
[0]: Assigning levels
[0]: Running program

See branch reactor-uc-flexpret-support for the LF compiler changes too

[magnmaeh]

I think this should be ready for review now

[github-actions[bot]]

Coverage after merging 22-add-platform-support-for-flexpret into main will be

35.11%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
external/nanopb
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