feat: add mode for selecting directories in file dialog

[sissbruecker]

Description

Adds a directory property which allows users to select a directory to upload from the upload file dialog. This recursively adds all files from the selected directory to the file list. Files are still filtered by the accept filter, and non-matching files will be rejected.

Some notes regarding the implementation:

• This only allows to select a single directory, it is not possible to select files or multiple directories. That is a browser limitation.
• All browsers that I've tested with show a confirmation dialog after selecting the directory where the user must confirm that they really want to upload all contents of that directory.
• It's still possible to drag and drop a combination of files and directories. I couldn't see a reason to disallow that, assuming that selecting a directory would just be a different shortcut to adding multiple files. Related to that, I didn't see the need to change the drop files label in this case.

Part of #857

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud