Detect uses of unsupported regexes early

[jjant]

As mentioned in this thread: #1998 (comment), Smithy accepts more regexes than the regex crate supports (the crate doesn't support look-behinds, etc) in the @pattern trait.

These unsupported regexes should be detected early, options include:

• During ${service}Builder::build() so that customers see an error as soon as they attempt to setup their server
• Generating tests that Regex::compile the regex, so builds fail if the regex is unsupported

[jjant]

Completed in #2058.