Add support for external C# script editors

I noticed the description mentions the Flatpak doesn't support external C# script editors. However, this is critical because Godot with C# support isn't really worth using if you can't use an external editor. The built-in editor for C# is very limited (it lacks autocompletion), so pretty much everyone using C# is using an external editor with it.

What are the roadblocks to using external C# editors that external GDScript editors don't have to deal with?

Start by reviewing the Flatpak description and the existing external GDScript editor support mentioned in the issue. Identify the C#-specific roadblocks to external editors and document a concrete implementation path; done requires an agreed scope rather than only an explanation of the problem.

Written by the indexing model from the issue text.