Now the operator is used for both the internal & external editor,
so there is no need for the caller to call this operator only when
the preferences are set.
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.
