[url_launcher][web] Better support for semantics in the Link widget

[mdebbar]

• Better support for semantics.
• Better support for clicks with a modifier key (e.g. cmd+click).
• More robust handling of events (can now correctly handle events received in a different order).
• Apply the target attribute on semantic links.
• Improve tests.

Fixes https://github.com/flutter/flutter/issues/143164 Fixes https://github.com/flutter/flutter/issues/146291

[harryterkelsen]

Re-opening this. It needs to land for https://github.com/flutter/flutter/issues/143164 to be fixed

[stuartmorgan-g]

From triage: What's the status of this issue? Is it still planned for further work?

[mdebbar]

Waiting for this commit to make it to stable.

[goderbauer]

(triage): https://github.com/flutter/flutter/commit/8d01fe0565e41ff1ad1b12bd6141fe636d1dc973 still hasn't made it to stable.

[ditman]

The tests are still conflicting, I'll try to resolve those.

[ditman]

I'm going to autosubmit this, since all the tests seem to be passing.