Implements the {project} placeholder, available when setting an external editor
in the project settings, via Editor > Editor Settings > Text Editor > External
for the c# external editor, under Editor > Editor Settings > Dotnet > Editor,
This allows passing the project folder as a command line argument when using a
custom external editor that isn't one of the available options.
Fixes#81845
(cherry picked from commit c01a47867b)
222cba2aab