Movie Writer does not work on Web Editor export

### Tested versions

4.3 web

### System information

firefox

### Issue description

On chromium, it would hang the entire system, on firefox, it at least complete and respond to the movie_quit_on_finish

So, basicaly, i tried several folders of the virtual filesystem, and none works. There is no file being written.
And, the terminal does not output any text regarding the movie being writen and stoped by the movie_quit_on_finish

So, the movie writer Is or Is Not implemented on the web editor (editor.godotengine.org)?

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Actualy, looking at the console, this is outputed:


### Steps to reproduce

Run the project on the online editor.

### Minimal reproduction project (MRP)

[Movie Writer Bug.zip](https://github.com/user-attachments/files/17356337/Movie.Writer.Bug.zip)

Start by reproducing the attached Movie Writer Bug.zip in editor.godotengine.org on Firefox and Chromium, checking the console and virtual filesystem. Determine whether Movie Writer is supported in the web editor; if it is, completion means producing the movie file and honoring movie_quit_on_finish without hanging.

Written by the indexing model from the issue text.