[flutter_markdown] Fix horizontal sizing of lists in MarkdownBody

[Giuspepe]

Fixes https://github.com/flutter/flutter/issues/108976

Markdown lists are now sized horizontally according to the existing fitContent parameter.

Before, only this behavior could be achieved when wrapping a MarkdownBody with a ColoredBox inside a Column:

Now this behavior can be achieved as well by setting fitContent to true:

Pre-launch Checklist

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• [x] I added new tests to check the change I am making, or this PR is test-exempt.
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