net/http: extended routing patterns

net/http: extended routing patterns

This is the first of several CLs implementing the proposal
for enhanced ServeMux routing, https://go.dev/issue/61410.

Define a type to represent extended routing patterns and a function to
parse a string into one.

Updates #61410.

Change-Id: I779689acf1f14b20d12c9264251f7dc002b68c49
Reviewed-on: https://go-review.googlesource.com/c/go/+/526815
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Eli Bendersky <eliben@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>