Fix a race between process death and stdio read

[lhchavez]

Previously we used to close the fd immediately after the process was terminated. That made lifetimes simple to reason about, but we accidentally introduced a very subtle bug: If bun hadn't read the entirety of the stdout/stderr of the child process, since Rust would close the file from under it, Bun would not be able to read the rest of the output, truncating it!

This change now adds an explicit close() and fd() functions so that the fd management becomes way more explicit than before. Tests were also updated to ensure that there are no leaked FDs, since now we lost the guarantee that everything was going to be cleaned up properly.

~/ruspty$ bun run build && bun test
$ napi build --platform --release
   Compiling replit_ruspty v1.0.0 (/home/lhchavez/ruspty)
    Finished release [optimized] target(s) in 15.19s
bun test v1.0.26 (c75e768a)

index.test.ts:
✓ PTY > spawns and exits [11.96ms]
✓ PTY > captures an exit code [7.81ms]
✓ PTY > can be written to [10.44ms]
✓ PTY > can be resized [9.45ms]
✓ PTY > respects working directory [7.24ms]
✓ PTY > respects env [110.81ms]
✓ PTY > works with Bun.read & Bun.write [15.14ms]
✓ PTY > doesn't break when executing non-existing binary [3.73ms]

 8 pass
 0 fail
 11 expect() calls
Ran 8 tests across 1 files. [334.00ms]