[FEATURE] Reverse tabstop

[half-adder]

Description of the Problem

On web forms, you can hold shift while pressing tab to move in reverse through web forms. Right now, we can only move forward in the equation, it would be nice if we could move back to the last tabstop and change what we entered.

Description of the Solution

Shift+Tab should allow for backwards tabstop movement.

Additional

Could be tricky since the actual tabstop characters ($0, ...) are now gone. Would have to remember their place in the string somehow.

[artisticat1]

I agree this would be useful to have. Should be possible using CodeMirror's undo history extension.