Add Codium support to C# external editors

[Calinou]

Selecting the VS Code option will now work if Codium is installed, picking it as the last option after attempting to run VS Code.

Not tested on any OS, so please test on macOS and Windows or Linux before merging.

• This closes Add VSCodium to the list of external C# editors godot-proposals#6518.

[paulloz]

This is probably fine. We could loop over a string[] { "com.microsoft.VSCode", "com.vscodium.codium" } instead of duplicating the whole block.
Would be nice if a Codium user could try it out, I'm not too keen on nuking my VSC setup to be able to test 😭.

[paulloz]

Finally tested. LGTM.

[Repiteo]

Thanks!