make `AsyncMatcher` public or create `AsyncCustomMatcher` to support custom, async matchers

[StephenWithPH]

It would be nice if this interface:

test/pkgs/matcher/lib/src/expect/async_matcher.dart

Lines 16 to 22 in 32bf9b7

	/// A matcher that does asynchronous computation.
	///
	/// Rather than implementing [matches], subclasses implement [matchAsync].
	/// [AsyncMatcher.matches] ensures that the test doesn't complete until the
	/// returned future completes, and [expect] returns a future that completes when
	/// the returned future completes so that tests can wait for it.
	abstract class AsyncMatcher extends Matcher {

... was either directly exposed in the package's public API or exposed via a new AsyncCustomMatcher whose use would closely follow https://pub.dev/documentation/matcher/latest/matcher/CustomMatcher-class.html.

For reference, there are cases in Flutter where the current, non-exported AsyncMatcher is used: https://github.com/search?q=repo%3Aflutter%2Fflutter+%22package%3Amatcher%2Fsrc%2Fexpect%2Fasync_matcher.dart%22&type=code