[flutter_markdown] Make custom table column alignments work when text wraps

[andrechalella]

This PR complements PR #7327 (Fix table column alignment), making the table column custom alignment keep working even when text wraps.

Ironically, setting Wrap.alignment wasn't enough, we needed to correct each Text.textAlign in the function _mergeInlineChildren(). Basically, this PR is a major refactor of said function, ensuring that the loop runs to completion even when there is a single widget in the children (thus remaking the widget with the correct textAlign).

This fixes flutter/flutter#160746.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• 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.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
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• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[andrechalella]

I fixed whatever failed checks I could, I feel the remaining ones have little to do with the PR itself.