Make shader editor menu position consistent with script editor

[lodetrick]

Currently, the shader editor has two modes, where the file menu button starts above the file list, then migrates to above the text editor when a file is selected, increasing the size of the file list vbox. This PR pulls the menu bars from the text editor above both the file list and the editor in order to be more consistent with the GDScript editor.

This PR also renames the properties left_panel -> main_container and main_split -> files_split to reflect this change in organization.

Iteration on #100287

Before:

After:

[Calinou]

Tested locally, it works as expected. Code looks good to me.

While this loses a bit of vertical screen real estate for the file list, this is more consistent with the script editor UX, so overall it's worth it.

[kitbdev]

When I tested it, I could not expand the shader editor view area.

[Repiteo]

Thanks!