proof-of-concept: preserve fixed width integer types

[Marcono1234]

For https://mail.openjdk.org/pipermail/jextract-dev/2026-January/002449.html (and previous thread messages)

This proof-of-concept is incomplete, but it demonstrates that (most likely) enough information is available to jextract to preserve the original types, and keep the C library portable.

For example the portable type int64_t could be preserved instead of turning it into the non-portable long.
When trying this locally with jtreesitter's jextract setup on Windows it seemed to emit portable code, without using types such as long or long long anymore. The only exception are C enums, which use int, but that appears to be portable (and since C23 it is also possible to specify a fixed type for an enum).

Most likely I will not complete this pull request; its main purpose is to highlight which code changes might be needed to preserve typedef information (but I am not very familiar with this project, so maybe there are missing places, and better ways to solve this).
Hopefully this is useful nonetheless.

Feel free to close this pull request.

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