fix: stream on sliced BunFile

[veera-sivarajan]

What does this PR do?

Stores sliced offsets inside FileReader for .stream() to use. Fixes #7057

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

• I included a test for the new code, or an existing test covers it
• I wrote TypeScript/JavaScript tests and they pass locally (bun-debug test test-file-name.test)

[Jarred-Sumner]

Thank you for this. The tests don’t hit the code path unfortunately. Bun implements fast paths for converting streams into native representations when using Response or Request. The chunks will need to be read manually

[veera-sivarajan]

Thanks for the quick review.

Yeah, I see that tests fail when checking streams on Response or Request, but I'm not sure if I understand this correctly. Should I reimplement BunFile's .stream() to read the chunks manually?