[flutter_markdown] Remove TaskListSyntax and rely on markdown native checkbox support - Fixes https://github.com/flutter/flutter/issues/107871

[timmaffett]

Fixes https://github.com/flutter/flutter/issues/107871

Note: This PR relies on the next Markdown to be released, as found in https://github.com/dart-lang/markdown/

This PR

1. Removes custom TaskListSyntax inline syntax as Markdown now supports checkboxes natively.
2. Changes the code in _buildCheckbox() slightly to just check for the existence of the 'checked' attribute to determine if a checkbox's state is checked.
3. Fix the test file list_test.dart to expect the indicatorForCheckedCheckBox and indicatorForUncheckedCheckBox codes to prefix the list item text, as Markdown now inserts these into the text when checkboxes are present.
4. [Temporary] changes pubspec.yaml to use markdown version from GitHub (temporary until next markdown release to pub.dev).

Pre-launch Checklist

• [X] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [X] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [X] I read and followed the [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [X] I signed the [CLA].
• [X] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [X] I listed at least one issue that this PR fixes in the description above.
• [X] I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• [X] I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
• [X] I updated/added relevant documentation (doc comments with ///).
• [X] I added new tests to check the change I am making, or this PR is [test-exempt].
• [X] All existing and new tests are passing.

[domesticmouse]

PTAL @srawlins

Tim, you have a couple of conflicts due to another PR landing. You'll need to update CHANGELOG.md to accomodate.

[timmaffett]

@domesticmouse I merged into the new changed master and updated this version to be called 0.6.10+4

[domesticmouse]

I'm happy to land this once we have a published version of the markdown package in the pubspec.yaml

[domesticmouse]

PTAL @stuartmorgan I believe this is now ready to land

[srawlins]

Note: the Ordered and Unordered task list item syntaxes are getting a big refactor soon. I don't think it should be breaking, but note just in case: https://github.com/dart-lang/markdown/pull/450

After this lands I plan on cutting markdown 6.0.1

[devoncarew]

Can we land this soon (or reformulate the PR and land something like it)? We haven't been able to roll the latest markdown into the dart sdk repo for ~3 months now; I believe we're waiting on this fix landing, this being published, and then the latest markdown and flutter_markdon packages rolling into the sdk simultaneously.