Remove deprecation of permission flags

[westbury]

As part of the switch-over to the vscode-compatible file service, the permission flags in FileAccess were deprecated. However these flags are just as useful for for both FileService.access and FileSystemProvider.access.

It took me a non-trivial amount of time to figure out what I was supposed to pass as mode to the access method. Hopefully the changes in this PR will make it clearer.

Although unrelated to permissions, I also noticed the comments for fsPath were misleading, missing the point that the conversion is done on the backend.

What it does

• removes the deprecation of the permission flags
• adds more useful comments to access and fsPath. These comments are copied over from the original comments in filesystem.ts.

How to test

I don't expect any of these changes to affect the runtime.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

LGTM 👍