fix(p2-shim/fs): correct read/write semantics

[MendyBerger]

Fixes openAt to check mutateDirectory.

Since we're checking flags correctly now, I added read/write flags to preopens.

[vados-cosmonic]

Thanks for this @MendyBerger . This support has been legitimately missing for 2 years at this point. thanks for finally adding it (with good comments and links to the spec!)

This is almost ready to go, only two things:

• fix(p2-shim/fs) -> fix(p2-shim) (we don't have sub targets yet! totally down to add that, but in another PR we can update the commit linting)
• Would you mind adding a regression test?

[vados-cosmonic]

@MendyBerger looks like fmt and lint are failing -- would you mind taking a look? You can run these locally with npm run lint or npm run fmt which should run across all subprojects!

[MendyBerger]

@vados-cosmonic know why ci didn't start?

[vados-cosmonic]

@MendyBerger They're configured not to for external contributors