[FSA] Add implementation for "write" permission mode

- Add `blink::mojom::FileSystemAccessPermissionMode::kWrite` and make
  the enum a standalone file to resolve dependency issues.
- Update `mojom::FileSystemAccess*Handle::GetPermissionStatus(bool)`
  and `mojom::FileSystemAccess*Handle::RequestPermission(bool)` to take
  `FileSystemAccessPermissionMode` instead of `bool writable`.
- Pipe down kWrite to content layer and update
  `FileSystemAccessHandleBase::DoRequestPermission()` and
  `FileSystemAccessHandleBase::DoGetPermissionStatus()` accordingly.
- The new permission mode is NOT surfaced to any IDL yet. And it should
  be no impact to existing FileSystemAccess APIs.

Bug: 40276567
Change-Id: Ic9888cf389a553dd06b6d9127f00a9d0087deae8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6767698
Reviewed-by: Austin Sullivan <asully@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Ming-Ying Chung <mych@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1491219}