Add export files feature to DevTools extension

[RuthySheffi]

This pull request introduces an export files feature to the DevTools extension.

Key updates include:

• Added export-handler logic to enable exporting scenes and assets directly from the DevTools panel.
• Integrated new UI elements and icons for export actions.
• Updated content script and panel logic to support export requests and communication.
• Added devtools in utils to copy build files and GLTFExporter to the extension folder.
• Updated the manifest for to run devtools.
• This feature streamlines the workflow for developers working with Three.js by making it easier to export and share assets during development.

Please review and let me know if any changes are needed!

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.4 78.67	336.49 78.38	-908 B -297 B
WebGPU	553.89 153.48	542.52 150.29	-11.38 kB -3.19 kB
WebGPU Nodes	553.24 153.32	541.87 150.13	-11.38 kB -3.19 kB

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	468.59 113.33	465.42 112.22	-3.17 kB -1.1 kB
WebGPU	628.86 170.19	615.29 166.29	-13.57 kB -3.89 kB
WebGPU Nodes	583.72 159.53	570.17 155.68	-13.55 kB -3.85 kB

[mrdoob]

That's quite a change...
It's going to take me quite a while to go through all the code.

Some quick notes:

1. Why was the build process introduced (build-devtools.js)?
2. I understand why the new permission "download" is needed, but why is the new permission "scripting" needed?
3. The CSS indentation style seems to have changed.

I don't think copying the files is a good idea...
Instead, I would import from jsdelivr instead and use import maps.

Also, can you add to the thread a screenshot or video of the feature?

[RuthySheffi]

Thank you for reviewing this PR and sharing your notes!

Why add the build process (build-devtools.js)?
The build process automates copying and adjusting files (like GLTFExporter) to ensure the DevTools panel uses the correct Three.js modules in the required formats (e.g., UMD). I'm open to refactoring if there's a better way to avoid file copying.

Why the new "scripting" permission?
Upon review, this permission isn't actually needed for the current implementation. I'll remove it in the next update.

CSS indentation style change:
Sorry for the inconsistency! I'll revert the CSS indentation to match the project's tabs.

Copying files vs. CDN/import maps:
Files were copied due to Chrome Extension CSP restrictions that limit using CDNs or import maps in DevTools panels. If there's a CSP-compliant approach available, I'd be glad to use it. Otherwise, local copies are necessary.

video:
https://github.com/user-attachments/assets/9bc437ad-0f9e-4f15-af27-5e6dc770e341

Thanks again! Let me know if you have any other suggestions or preferences.