External editor improvements and fixes

Notable changes:

- Now ScriptLanguages have the option to override the global external editor setting.
If `ScriptLanguage::open_in_external_editor()` returns `ERR_UNAVAILABLE` (which it does by default), then the global external editor option will be used.
- Added formatting to the external editor execution arguments. Now it's possible to write something like this: `{project} -g {file}:{line}:{col}`.
- `VisualScript::get_member_line()` now can return the line of functions (well, it returns the id of the _Function_ node of the function). I guess there is nothing else we can get a "line" from.

Fixes:

- Fixes a bug where `ScriptEditor::script_goto_method()` would not work if the script is not already open in the built-in editor.
- Fixes wrong DEFVAL for `cursor_set_column` and `cursor_set_line` in TextEdit.
- `Script::get_member_line()` now returns -1 ("found nothing") by default.

editor/connections_dialog.cpp

@@ -767,7 +767,7 @@ void ConnectionsDock::_something_activated() {
 
 		Ref<Script> script = c.target->get_script();
 
-		if (script.is_valid() && ScriptEditor::get_singleton()->script_go_to_method(script, c.method)) {
+		if (script.is_valid() && ScriptEditor::get_singleton()->script_goto_method(script, c.method)) {
 			editor->call("_editor_select", EditorNode::EDITOR_SCRIPT);
 		}
 	}