Add support for extension types

[srujzs]

Corresponds to the syntax defined in https://github.com/dart-lang/language/blob/main/accepted/future-releases/extension-types/feature-specification.md#syntax.

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

[jakemac53]

@srujzs fwiw force pushing in GitHub doesn't work well because it destroys all history (I can't tell what you actually changed in that last push). If you can do a merge from master instead in the future that will help me out :).

[jakemac53]

LGTM modulo the formatter change

[srujzs]

fwiw force pushing in GitHub doesn't work well because it destroys all history (I can't tell what you actually changed in that last push).

Ah sorry about that! I've been pushing to avoid unnecessary commits from polluting the branch but messing with the review is way worse.

[jakemac53]

Ah sorry about that! I've been pushing to avoid unnecessary commits from polluting the branch but messing with the review is way worse.

No worries, really it is just specific to GitHub and how its reviews work. Fwiw when we merge we squash/merge so its always just one commit. So you don't have to worry about branch history really (and it is useful for reviewing so you can see diffs since last review).