This PR adds framework support required for Material Symbols (design doc, #102560).

Add 4 members to Icon: fill, weight, grade, and opticalSize. Under the hood, pass these to the engine using the FontVariation API. No guarantees are provided:

• If used with a font which does not support these axes => no effect
• No bounds checking for values against the font ⇒ no effect outside variable font's supported range

Add 4 corresponding members to IconThemeData to customize all icons in a given widget subtree.

Also:

• debugFillProperties is completed
• properties are reordered for consistency
• documentation is improved

Tests

• Basic bounds checking
• Instantiation
• Fallbacks

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• 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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