Fixes edge cases where the `startsWith` that was used previously would return a false positive on a resolved path like `foo.zig` when the resolved root was `foo`. Before this commit, such a path would be treated as a sub path of 'foo' with a resolved sub file path of 'zig' (and the `.` would be assumed to be a path separator). After this commit, `foo.zig` will be correctly treated as outside of the root of `foo`.

Closes #18355