[WIP] Use Express-style path matching

[taion]

This is a remake of remix-run/react-router#3105.

This is not quite ready for release, as I want to make this an opt-in feature guarded behind a deprecation for now, as well as to update the documentation and examples as appropriate.

This PR changes the pattern matching algorithm from the home-grown React Router to path-to-regexp, which is used by Express and a number of other JavaScript routing frameworks.

The biggest advantages of this change are:

• No more need to maintain a bespoke pattern matching algorithm, and instead use a syntax consistent with the rest of the JavaScript ecosystem
• Support for regex-qualified route parameters, for constraining route patterns (Customizable URL matching rules remix-run/react-router#2286)
• Dramatically increased performance, though for reasons somewhat unclear to me (Performance regression on route matching for sites with 20+ routes remix-run/react-router#3215, providing a more optimal implementation than is available on React Router)

This will come with a few breaking changes once the feature is enabled:

• Optional segments must be specified as /:foo? as with Express, rather than (/:foo) as with RR v1+ (but this is the syntax in v0.13.x)
• Splats now come through as numbered parameters, rather than under splat
• Repeated named parameters will take the last value, rather than collapsing into an array
• Non-greedy splats are no longer supported; * is a greedy splat

In other words, Express's path matching works slightly differently to React Router's.

I expect that for the majority of applications, this should be fully drop-in. For the rest, the migration work will be relatively limited.

[taion]

And for what it's worth, I don't like the Express syntax for optional segments, but I feel like consistency here trumps my aesthetic preferences.

[taion]

My current thoughts for backward compatibility strategy:

• Allow setting patternMatcher={version} on a route-by-route basis
• Allow a top-level global static default flag for changing the pattern matcher version
• In production mode, use the specified pattern matcher
• In development mode, if the old pattern matcher is specified, run both pattern matchers, and warn if the results differ