[Dropzone] Allow for multiple file uploads at once

[Dooij]

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Documentation?	yes
Issues	Fix##7 Fix ##2567
License	MIT

Adds proper multiple support to UX Dropzone.

A native <input type="file" multiple> replaces its selection on every pick and offers no way to remove a single file, so the Stimulus controller now owns the selection in a DataTransfer: it accumulates files across successive picks (files with the same name, size and updatedAt are seen as the same file and not added more than once), writes the result back to input.files so the form still submits everything, previews each file, and lets the user remove files individually.

Behavior is triggered by enabling the Typesmultiple option (from FileType).

Usage:

$builder->add('photos', DropzoneType::class, [
    'multiple' => true,
]);

A dropzone:remove event (the removed File in event.detail) has been added.

Tests: vitest (accumulate / dedupe / remove), a Playwright browser test using real setInputFiles, and a PHP render test for the multiple markup.

[github-actions]

📊 Packages dist files size difference

Thanks for the PR! Here is the difference in size of the packages dist files between the base branch and the PR.
Please review the changes and make sure they are expected.

File	Before (Size / Gzip)	After (Size / Gzip)
Dropzone
controller.d.ts	688 B / 332 B	1.06 kB+58% 📈 / 459 B+38% 📈
controller.js	2.72 kB / 797 B	4.88 kB+80% 📈 / 1.28 kB+65% 📈
style.min.css	945 B / 471 B	1.52 kB+65% 📈 / 590 B+25% 📈