Support for more desktop keyboard keys/shortcuts.

Switch document generation to use the snippets package instead of the snippets code in the Flutter repo. In the process, some bugs in sample code analysis have been fixed, and I've fixed some more errors in the samples.

This will allow the snippets package to be developed separately from the Flutter repo, and reduce the code in the Flutter repo.

The snippets code is deleted in this PR.

I also converted some comments in the snippet templates to be regular comments instead of doc comments, because having a doc comment block before the imports causes the Dart import sorter to lose the comment. They should have been regular comments in the first place.

The snippets package resides in the assets-for-api-docs repo.

The sample analysis has also been converted to be run in parallel, and I've bumped the Dartdoc version to 1.0.2.

This reverts commit 724c0eb6.

This will still run the workflow in forks but it will exit immediately
if the repository is not flutter/flutter.

Bug: https://github.com/flutter/flutter/issues/87573

This reverts commit ccf6f035.

New impl

Remove temp

Format

Added a check to the analyzer script to detect skipped tests that aren't commented.

The comment following the `skip` parameter should include either a link to a
github issue tracking the reenabling of the test, or a '[intended]' tag with
a brief description of why the test should never be enabled for the given
condition.

* Updated skipped tests for flutter_tools.

* Turned the flaky overall_experience test off as it was still failing on CI.

* Reland "update ScrollMetricsNotification (#87421)"

This reverts commit 0f465e5b.

* fix test

Python 2 has been end-of-life'd since January 2020, all Python scripts
on which Flutter depends have been migrated to Python 3. This changes
documentation and remaining invocations to explicitly invoke python3.

Issue: https://github.com/flutter/flutter/issues/83043