scripting tool: Use project buffers in `io.open` (#26425)

This PR makes `io.open` use our own implementation again, but instead of
the real filesystem, it will now use the project's to check file
metadata and perform read and writes using project buffers.

This also cleans up the `io.open` implementation by splitting it into
multiple methods, adds tests for various File I/O patterns, and fixes a
few bugs in read formats.

Release Notes:

- N/A