Add support for named wildcards

[randomdevpete]

It felt weird and inconsistent that wildcard captures cannot be named. As I am using url-pattern in a routing library I want to make the interface simple for catch-all routes and a named wildcard seems a far better API than making the user map _ to something more friendly later.

This patch allows the syntax *:name to name a wildcard rather than using the magic key _. Wildcards can still be unnamed so this does not break any existing functionality.

This syntax was chosen because it was in any case ambiguous under the old parsing rules, this clears up an ambiguity whilst adding useful new behaviour.

Tests have been added but docs have not been updated yet, if this functionality is desired in master then I will write some updates to the readme as well.

[allmarkedup]

I'd love this to be merged in as I to am exposing this as a routing lib and suffering from the same issue. Allowing users to map to a named param rather than _ would be a major UX improvement for me.

[randomdevpete]

@snd said in #45 this could be merged but was some time ago and haven't heard anything since.

[randomdevpete]

@allmarkedup btw in my own lib I just made a a custom build and copied that directly into my package for the time being, if you want to use my build it's here: https://github.com/downplay/jarl-react/tree/master/packages/jarl-react/source/vendor