[material_ui] Set up gen_defaults sub-directory

[elliette]

Work towards flutter/flutter#186906

Adds the initial gen_defaults sub-directory with the initial TokenTemplate and generator. This is simply the initial scaffold, we will need to add additional helper functions as necessary as we begin adding the actual templates.

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Code Review

This pull request introduces a new 'Token Defaults Generator' tool in the material_ui package, consisting of a command-line script, a base template class for code generation, and unit tests. Review feedback identifies a discrepancy between the documented and implemented output paths, a typo in the SDK version constraint which is incompatible with the workspace resolution feature, and a potential runtime error when creating files in non-existent directories.

[QuncCccccc]

Do you think whether naming the folder gen_defaults would be better? Since currently the tokens we exported here also include M3 widget tokens, the new gen_defaults could be used for M3 token update (if needed).

[QuncCccccc]

Overall LGTM! Just left some comments.

[elliette]

Do you think whether naming the folder gen_defaults would be better? Since currently the tokens we exported here also include M3 widget tokens, the new gen_defaults could be used for M3 token update (if needed).

Good point, renamed to gen_defaults. I also set it up so the M3 and M3E generated pipelines can both use gen_defaults, but get outputted to slightly different directories:

M3

• Requires the M3TokenTemplate
• Outputs generated code to material_ui/lib/src/generated directory
• Expects parent file to be in material_ui/lib/src

M3E

• Requires the M3ETokenTemplate
• Outputs generated code to material_ui/lib/src/generated/material_3_expressive
• Expects parent file to be in material_ui/lib/src/material_3_expressive

[QuncCccccc]

LGTM! Thanks for set it up!🎉

[QuncCccccc]

get outputted to slightly different directories...

The output structure looks good to me! Thanks for the update!