Prevent the internal Script Editor from reloading

when the external Editor is active in the settings.

This prevents a crash from different race conditions
from edititing files in the external editor while the internal editor
tries to reload open file tabs.

Some conditions that cause the crash where:
* Syntax highlighting (gdscript_highlighter.cpp, syntax_highlighter.cpp)
* Code analysis (code_edit.cpp)
* ... and more

editor/plugins/script_editor_plugin.h

@@ -396,6 +396,7 @@ class ScriptEditor : public PanelContainer {
 	bool open_textfile_after_create = true;
 	bool trim_trailing_whitespace_on_save;
 	bool convert_indent_on_save;
+	bool external_editor_active;
 
 	void _goto_script_line2(int p_line);
 	void _goto_script_line(Ref<RefCounted> p_script, int p_line);