Custom monitors can be added/removed/checked using `Performance.add_custom_monitor`/`Performance.remove_custom_monitor`/`Performance.has_custom_monitor`

The value can be viewed in the `Monitor` tab of Debugger.

Text before `/` is used to categorize the custom monitor.

`EditorPerformanceProfiler` class is created to separate logic from `ScriptEditorDebugger`

User can click on the graph of monitors to read the value at that point.

Graph includes intermediate base lines.