Thank you. and everyone else.
Turns out I had forked the repository and cloned from the head of the master branch (Which at the time, was Godot 4.x).
I deleted my local repository of godot, and this time directly cloned the 3.x branch. The syntax for tool in Godot 4 had changed, which was the cause of my issue.
Also, an additional note for any future users. If you want to add Editor functionality, work with the EditorModule class, most of the main classed won't respond in the editor even if you add the tool keyword (e.g. Node, Control, CanvasItem, Node2D).