[material_ui] Add Material 3 Expressive IconButton

[QuncCccccc]

This PR is to add Material 3 Expressive IconButton support behind an explicit component-level opt-in.

This PR adds:

• 3 new ButtonStyle properties used by the IconButton M3E migration:
  • sizeVariant
  • iconButtonWidth
  • shapeVariant
• IconButtonThemeData.variant so IconButtons can opt into StyleVariant.material3Expressive.
• IconButtonTemplateM3E to generate IconButton defaults from the M3E token data.
• Adds the generated M3E IconButton defaults in:
  • packages/material_ui/lib/src/generated/icon_button_defaults_m3e.g.dart

TODO:

• I'll add a separate PR/demo showing the Material 3 Expressive IconButton variants.
• Will also update IconButton shape morphing to use physics-based motion animation.

Pre-launch 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] 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].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.

[QuncCccccc]

This PR is still trying to merge into m3e_migration branch. I'll create another PR targeting to land in main once we move all previous m3e_migration commits to main, and update changelog. But feel free to review the IconButton change for now:)

[elliette]

What happens if the Material specs are updated? It seems possible that this documentation could get out of date

[QuncCccccc]

Yes, they will change the number but not super frequent. I usually just keep an eye on the new email from material-token-announce to see whether any changes that have been made and we need to pay attention.

Maybe later we can use agent to do regular check on the accuracy of documentation? Just brainstorm haha.

[elliette]

oops good catch!