[two_dimensional_scrollables] Add alignment to TreeView and TableView

[Piinks]

This PR implements declarative alignment for the content of TableView and TreeView widgets when that content is smaller than the viewport extent. Users can now use the alignment property to center or otherwise position the entire table/tree within the viewport.
Alignment correctly reverts to start for axes that exceed viewport dimensions.

Fixes flutter/flutter#170349

• TableView

  • Added an alignment property of type AlignmentGeometry (defaults to Alignment.topLeft).
  • Full support for both horizontal and vertical alignment.
  • Supports AlignmentDirectional to correctly handle TextDirection (LTR/RTL).
  • Works with pinned rows and columns, as well as reversed axis directions.

• TreeView

  • Added an alignment property of type AlignmentGeometry (defaults to Alignment.topLeft).
  • Caveat: Currently only supports the vertical component of the alignment. The tree remains
    aligned to the horizontal "start" to maintain consistent indentation logic and avoid layout
    jumps caused by dynamic, lazily-loaded node widths.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• 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 followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request introduces an alignment property to TableView and TreeView widgets, allowing content to be aligned within the viewport when its extent is smaller than the viewport. The implementation involves adding the alignment property to the widget, viewport, and render object constructors, calculating horizontal and vertical alignment offsets in the render objects, and applying these offsets during layout and painting, considering pinned elements and reversed scroll directions. The package version has been updated to 0.4.0, and new comprehensive tests have been added for both TableView and TreeView to validate the alignment functionality across various scenarios. There is no feedback to provide.

[Hari-07]

LGTM, but maybe we can have a test for the default case too? So that the default behaviour does not change unexpectedly in the future

[Piinks]

Good call! Thanks! I added the default test cases.