fix: toggle_operator() support for languages that target a childless node

[alexpw]

Issue reported here: https://github.com/CKolkey/ts-node-action/issues/29#issuecomment-1462864159

This fixes toggle_operator() in Julia and any language that act on the equivalent of an operator node that has no children.

Includes a short julia spec that tests this case.

[CKolkey]

You just have to add julia to the treesitter install step to fix this

[CKolkey]

Awesome :D