refactor(packages/codegen): split categories into separate file (#26083)

Pure refactor. Move the category consts (`CAT_IDENT` etc) into a separate file.

2 reasons:

1. `write.ts` had got too large.
2. `index.ts`, which is not part of printer builds, imports `CAT_OTHER`. Importing it from a file containing only consts avoids any problems with minifier not being able to recognise dead code and remove it.