[url_launcher_web] Added support for mailto in url_launcher_web plugin

[ianyon]

Description

Adds support for opening mailto links in url_launcher_web using the default browser behaviour.

Related Issues

• Resolves: url_launcher for web should support the mailto scheme flutter#49529

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[ianyon]

@googlebot I fixed it.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[otopba]

@amirh
LGTM

[ditman]

LGTM. @hterkelsen? (Do you know what's that branch-guard check that is failing?)

[amirh]

branch-guard is supposed to guard against landing PRs while the tree is red, it was introduced in #2492 and reverted quickly after (as it had a bug). I'm not sure why it shows here, feel free to ignore it for now as long as you manually check that the tree is green (rebasing may make that check go away).

[ditman]

@amirh makes sense, thanks for the explanation! Maybe this PR was created while 2492 was active.

[ditman]

The tree is currently green, I'm merging this as per @amirh comment.

[ditman]

Tagged and published to pub.dev: https://pub.dev/packages/url_launcher_web/versions/0.1.1