fix: gracefully handle forbidden filesystem access

[brillout]

Description

Prior to this PR, URLs like /root make Vite throw an error.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

Note: just a detail, but about the "is self explanatory" in the Description, IMO you could avoid that line and directly add the Additional Context line instead there.

[brillout]

I added unit tests and rewrote isFileReadable() which was buggy: if you revert the new implementation you'll see the unit test fail.

I also removed comments that weren't useful. (The comments were linking to #2051 which doesn't provide much information.)

The new implementation only uses fs.accessSync() (instead of fs.statSync()) because it's semantically more accurate, which is also important for performance.

[patak-dev]

Thanks @brillout! This looks good to me now.