Create new movie file if already exists

[morganwesemann]

This is branched off of #71420, which I think has been abandoned after asking on the PR back in March.

This PR implements the requested feedback on top of the original changes from the original PR to create a new movie file if the path already exists instead of overwriting. Thank you to the original author for opening the original PR, this is a feature I've been wanting!

This is the sample project I created for testing the code: https://github.com/morganwesemann/godot-movie-maker-test

This will close godotengine/godot-proposals#5989.

Here's what the setting looks like which defaults to true.

Multiple files for both .avi and .png extensions in the project movie file settings:

Log output showing correct filename:

Movie Maker mode enabled, recording movie at 60 FPS...
----------------
Done recording movie at path: res://Movie.avi
25 frames at 60 FPS (movie length: 00:00:00), recorded in 00:00:03 (0% of real-time speed).
CPU time: 0.00 seconds (average: 0.11 ms/frame)
GPU time: 0.00 seconds (average: 0.01 ms/frame)
----------------

< more logs >

Movie Maker mode enabled, recording movie at 60 FPS...
----------------
Done recording movie at path: res://Movie_1.avi
31 frames at 60 FPS (movie length: 00:00:00), recorded in 00:00:03 (0% of real-time speed).
CPU time: 0.00 seconds (average: 0.10 ms/frame)
GPU time: 0.00 seconds (average: 0.01 ms/frame)
----------------