[material_ui] Standardize M3 and M3E token template base class names - #12209

…pt (#12122)

~Will land #12209 first.~

Fixes flutter/flutter#188397

Initially gen_defaults script and its templates were used to handle JSON
tokens. After we fix the token pipeline and make it possible to get M3E
tokens, we use Dart constants and re-created the gen_defaults in
material_ui. After decoupling, the old gen_defaults will be deprecated
soon.

This PR is to:
* create a template for M3 `AppBar` so we can use new script to generate
M3 defaults.
* move the generated code to `generated/` folder so all M3 and M3E
generated code will be in the same location.

## Pre-Review Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
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responsibilities, or I am not using AI tools.
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- [x] I read and followed the [relevant style guides] and ran [the
auto-formatter].
- [x] I signed the [CLA].
- [x] The title of the PR starts with the name of the package surrounded
by square brackets, e.g. `[shared_preferences]`
- [x] 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
under[^1].
- [x] I updated/added any relevant documentation (doc comments with
`///`).
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…12675)

Work towards flutter/flutter#191088

**Ports over #12209 which landed on the `m3e_migration` feature branch.**

### Original PR description

As title. Related to flutter/flutter#187899

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). 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.

[^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.

…lutter#12675)

Work towards flutter/flutter#191088

**Ports over flutter#12209 which landed on the `m3e_migration` feature branch.**

### Original PR description

As title. Related to flutter/flutter#187899

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). 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.

[^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.

…lutter#12675)

Work towards flutter/flutter#191088

**Ports over flutter#12209 which landed on the `m3e_migration` feature branch.**

### Original PR description

As title. Related to flutter/flutter#187899

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). 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.

[^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.