Add `ECMAScriptRegex`, LibRegex's C++ facade for ECMAScript regexes.
The facade owns compilation, execution, captures, named groups, and
error translation for the Rust backend, which lets callers stop
depending on the legacy parser and matcher types directly. Use it in the
remaining non-LibJS callers: URLPattern, HTML input pattern handling,
and the places in LibHTTP that only needed token validation.
Where a full regex engine was unnecessary, replace those call sites with
direct character checks. Also update focused LibURL, LibHTTP, and WPT
coverage for the migrated callers and corrected surrogate handling.
This provides the infrastructure for taking a part list from the
pattern parser and generating the actual regexp object which is
used for matching against URLs from the pattern.
A URL pattern consists of components such as the 'port', 'password'
'hostname', etc. A component is compiled from the input to the
URLPattern constructor and is what is used for matching against
URLs to produce a match result.
This is also where the regex dependency is introduced into LibURL
to support the URLPattern implementation.
