Add a user preference to set up a custom text editor for editing text
files with the "Edit Source" action in the UI context menu.
- An operator TEXT_OT_jump_to_file_at_point has been added.
- A custom editor can be set in the user preferences.
- A preset has been included for "Visual Studio Code".
- When the editor is not set, use Blender's internal editor.
Ref !108299.
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