Unable to generate fbs files using the --proto flag

[le-michael]

When trying to generate an fbs file from a proto file I get the error

no generator registered

Running ./tests/prototest/GenerateProtoGoldens.sh fails with the same error.

This could be related to the recent CodeGenerator changes (#7802)

[le-michael]

We'll need to migrate idl_gen_fbs to use the new CodeGenerator class.
https://github.com/google/flatbuffers/blob/master/src/idl_gen_fbs.cpp

Then register it.
https://github.com/google/flatbuffers/blob/master/src/flatc_main.cpp#L82