parser: handle underline_in_numbers feature

[taylorplewe]

What's changing

Conditionally accept underscores in numbers (e.g. %0000_0101) if the user has enabled the feature above the current line

Motivation for change

Currently underscores are parsed as invalid no matter what:

Background/Notes

As this is the first issue of this kind, it will probably involve creating some kind of features structure to keep track of which features are turned on