Build: Normalize JSX file extensions

[ciampo]

Part of #80855. Follows #81040 and preserves #80123.

What?

Rename tracked JavaScript files that contain JSX to .jsx, including related snapshots, references, and changelogs.

Why?

Make JSX-bearing files explicit to Vite and other tooling as a foundation for the test migration.

How?

• Of 1,506 changed paths, 1,306 are exact renames and 13 are renames with related edits. The remaining 187 update references, tooling, and changelogs.
• Keep wp-build compatibility for JSX in .js, while ESLint continues to require new unsuppressed JSX files to use .tsx.
• Update import resolution so built JavaScript exports map to source without hiding unresolved non-build exports.

Supported package exports and build output are unchanged. Unsupported internal src paths now use .jsx.

Testing Instructions

1. Run npm run lint:js.
2. Run npm run typecheck.
3. Run npm run test:unit.
4. Run npm run test:unit:vitest.
5. Run npm run test:unit:routing.
6. Run npm run build.

Testing Instructions for Keyboard

Not applicable. This PR does not change UI behavior.

Use of AI Tools

Codex assisted with the mechanical rename, rebase, review fixes, and verification. The author reviewed the changes and test output.

[github-actions]

Size Change: 0 B

Total Size: 7.91 MB

_{compressed-size-action}

[github-actions]

Flaky tests detected in 0bddaab.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/33074831205
📝 Reported tests:

Should navigate inner blocks with arrow keys in /test/e2e/specs/editor/various/writing-flow.spec.js, passed after 1 failed attempt.

TimeoutError: locator.click: Timeout 10000ms exceeded.
Call log:
  - waiting for getByRole('listbox', { name: 'Blocks' }).getByRole('option', { name: 'Paragraph' })

    at WritingFlowUtils.addDemoContent (/home/runner/work/gutenberg/gutenberg/test/e2e/specs/editor/various/writing-flow.spec.js:1482:5)
    at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/editor/various/writing-flow.spec.js:30:3

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: manzoorwanijk <manzoorwanijk@git.wordpress.org>
Co-authored-by: aduth <aduth@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[manzoorwanijk]

Looks fine to me

[aduth]

• Keep wp-build compatibility for JSX in .js, while ESLint continues to require new unsuppressed JSX files to use .tsx.

Would we ever remove this? I feel like we added a bunch of hacks about supporting JSX in .js files just because we didn't have this normalization but that we'd be better off without them. Understanding that this potentially has a downstream impact, but I think we're in a strong position to be opinionated about this (following the ecosystem's lead) as a documented breaking change.

[aduth]

This should let us remove some of the artificial supports we added to Storybook as well: #82192