Better function signature help

[genius257]

Currently it does not know what parameter you are on, but guesses based on when a comma is pressed.

It is tricky when the parser MIGHT detect a syntax error and not be able to calculate it based on AST tree information.

Another problem with the AST is there is no information about the position of the comma's between the parameters being entered, so currently it is tricky to know when moving with the arrow keys if the threshold between two parameters are crossed.

I will haft to look at how other extensions handle this, and see if a non janky solution is currently possible.