refactor(packages/codegen): simplify TSDown plugin (#25969)

The TSDown `unmap_writes` plugin was a bit sloppy. It was over-engineered, doing multiple passes over the AST, printing it and re-parsing it. Condense this all down to a single AST pass.

In addition:

1\. Check the declarations of listed functions to make sure that `arity` accurately matches the actual declarations.
2\. Rename `plain` to `rename` - that better describes what this property is.

This does not alter the output code.