Support negative decimal numbers

[AndreasArvidsson]

We support negative numbers and decibel numbers, but not the combination. For negative numbers we have a specific signed number capture. Do we want to make a signed decimal capture or should we just extend the existing decimal capture with support for negative numbers?

[FireChickenProductivity]

I would prefer a separate signed decimal capture for the sake of user custom grammars using these captures in other contexts outside of prose dictation where an unsigned capture may be desirable. I have used unsigned decimal numbers as arguments to drawing commands.

I could probably work on this sometime during the next few weeks if no one else wants to take it.