feature #1242 [DX] Update .gitattributes & .gitignores files (smnandre)

This PR was squashed before being merged into the 2.x branch.

Discussion
----------

[DX] Update .gitattributes & .gitignores files

StimulusBundle did not have a `.gitattributes` file, packaging all the tests code (see: [symfony/stimulus-bundle](https://cdn.jsdelivr.net/gh/symfony/stimulus-bundle/)  VS. [symfony/ux-twig-component](https://cdn.jsdelivr.net/gh/symfony/ux-twig-component/)

| NOK | OK |
| - | - |
| ![stimulus-bundle](https://github.com/symfony/ux/assets/1359581/c82d78dd-535c-4f17-a96a-70895f167c43) | ![twig-component](https://github.com/symfony/ux/assets/1359581/76bacad4-7c39-4fce-a389-bfe3c2a07fe8) |

I used the occasion to check / refresh all the packages `.gitignore` and `.gitattributes` and
* add missing phpunit.xml.dist (once)
* remove unnecessary .gitignore files
* simplify export rules about assets/src files

Commits
-------

35832abfc3 [DX] Update .gitattributes & .gitignores files

Author: weaverryan

.gitattributes

@@ -1,9 +1,7 @@
 /.gitattributes export-ignore
 /.gitignore export-ignore
 /.symfony.bundle.yaml export-ignore
-/phpunit.xml.dist export-ignore
-/assets/.gitignore export-ignore
-/assets/src/**/*.ts export-ignore
+/assets/src export-ignore
 /assets/test export-ignore
 /phpunit.xml.dist export-ignore
 /tests export-ignore