`parseRoutePattern` plugin hook

[aleclarson]

We currently use the regexparam library to parse route patterns, but it would be cool if plugins can override this and even provide support for values other than strings (eg: RegExp or options object).

https://github.com/alloc/saus/blob/052b3cc847c131bde25a2c463c59f858ad67c7c4/src/core/routes.ts#L96

The plugin hooks would work a lot like resolveId does.

const plugin: import("saus").Plugin = {
  // The hook receives a route pattern and the route's config object,
  // which should also be customizable through the plugin API.
  parseRoutePattern(route, config) {
	if (route instanceof RegExp) {
	  return {
		// Match against a page path.
		test: path => route.test(path),
		// Render a page path for this route.
		render: params => {
		  // TODO: inject our `params` into the `route.source` somehow
		},
	  }
	}

	// Return falsy to defer to the next plugin hook.
	return null
  }
}