Media: Re-introduce client-side media processing feature

[adamsilverstein]

Description:

The client-side media processing feature (wasm-vips) was removed in [62081] (see #64906) because the VIPS WASM worker added too much build size overhead for the value provided at the time.

Once WordPress 7.1 has forked, this ticket re-introduces the feature for the next release cycle.

Changes Restored

Reverts the removal, restoring all:

• PHP functions for client-side media processing (wp-includes/media.php)
• REST API endpoints for attachment processing (class-wp-rest-attachments-controller.php)
• Cross-origin isolation infrastructure (COOP/COEP headers in class-wp-rest-server.php)
• VIPS script module registration (script-modules.php)
• Media templates (media-template.php)
• Default filter hooks (default-filters.php)
• Build configuration (Gruntfile.js, tools/gutenberg/copy.js)
• Associated PHPUnit and QUnit tests

14 files changed, ~1,625 lines restored.

PR

​PR #11324

## Summary

• Reverts the removal in [62081] now that WordPress 7.1 has forked
• Restores all PHP functions, REST API endpoints, cross-origin isolation infrastructure, VIPS script module handling, build configuration, and associated tests
• 14 files restored, ~1,625 lines

## Test plan

• [ ] CI passes (coding standards, PHPUnit, JS tests)
• [ ] Verify client-side media processing functions are restored in wp-includes/media.php
• [ ] Verify REST API endpoints for attachments are restored
• [ ] Verify cross-origin isolation test suite passes
• [ ] Verify VIPS script module registration is restored

Props adamsilverstein, jorbin.
See #64906.