Add a syntax definition for our format that will be used in the insert panel

[FichteFoll]

You can set syntaxes for input panels (afaik), so we should probably add a definition for our format language and highlight the input with it.

There's a EBNF-like syntax specification for this in the repo: https://github.com/jbrooksuk/InsertNums/blob/master/format_syntax.txt

[FichteFoll]

This is likely to conflict with #28 because I don't think you can reliably set a syntax for the quick panel since you can't acquire a handle (or id) for its view. We could do display some syntax help there and show how the current expression would be split into the different sections, however.

Or we just separate into multiple input handlers for each section which would be easier to use with fewer special operators to remember but also prevent you from quickly editing a previous section without losing what you have specified later in the pipeline …