[Nacl-HCl]

Fixes #955.

Url::set_scheme rejected every change to the file scheme for URLs with an
authority (new_scheme_type.is_file() && self.has_authority()), which does not
match the URL Standard. Under the scheme-state override rules, switching to
file must only be rejected when the URL includes credentials or has a
non-null port, and switching away from file must be rejected when the
file URL's host is empty or null. The no-host guard below the checks also
treated file like the other special schemes, even though file does not
require a host, which kept file URLs without a host from retaining the
file scheme.

With this change:

• Url::parse("file:///path/to/file.js")?.set_scheme("file") returns Ok(())
  (the case reported in The incorrect implementation of set_scheme for file:// #955).
• http://localhost/foo (no credentials, no port) can switch to file,
  while URLs with credentials or a port still cannot.
• A file URL with an empty host still cannot switch to another scheme.

Tests

Added regression tests in url/tests/unit.rs for switching to file with
credentials, switching to file without a port or credentials, setting file
on a file URL, and switching away from file with an empty and a non-empty
host. Verified locally:

• cargo test -p url — unit (72), WPT (67) and doc tests (67) all pass
• cargo test --workspace — all pass
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --all --check — clean
• cargo +1.63.0 test -p url --test unit with ci-lockfiles/1.63.Cargo.lock
  — 72 passed (MSRV)