Support git tag_pattern from Dart 3.9.0

[spydon]

In Dart 3.9.0 the tag_pattern for git dependencies was introduced. (https://dart.dev/tools/pub/dependencies#git-packages)
But it doesn't seem like there is support for that in the pubspec_parse package yet?

[devoncarew]

Ah, good catch. pubspec_parse doesn't support this yet, but we would accept PRs adding support for it. cc @sigurdm @szakarias