[TS/JS] Introduce documentation and helper scripts on how to produce single file builds for generated code

[bjornharrtell]

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This is similar to https://github.com/google/flatbuffers/pull/7449 but for generated source.

It would produce output that is more or less compatible with what we had in 1.x generated code.

However to be compatible https://github.com/google/flatbuffers/issues/7448 needs to be adressed.