Add support for signed/fixed-point fields

[darsor]

Description of change

Add support for the is_signed, intwidth, and fracwidth properties as discussed in SystemRDL/#188.

This is an initial implementation. I don't claim that all the edge cases are handled (though I'm not aware of any that aren't) or that the output looks great. I also don't know any javascript so Copilot helped out quite a bit.

List of changes:

• Added is_signed, fracwidth, and intwidth UDPs (implementation copied from PeakRDL-regblock)
• Added "R" (Real) radix for fixed-point fields
• The "D" radix field values are parsed/displayed as signed for signed fields
• The "D" radix is the default radix for signed fields
• The "R" radix is the default radix for fixed-point fields
• Out-of-bounds parsed field values for all radices are clamped to the nearest valid field value for the register value
  • This behavior makes sense for numeric fields, but may not be what you want for everything
• Added field value display below the entry box for signed/fixedpoint fields

TODO (but need feedback/discussion before implementing):

• Find an appealing way to include signed/fixedpoint information in the table of fields. Where's the most appropriate place?
• Documentation. Likely all in the turbo encabulator example regfile?
• Discuss if the new saturation behavior is appropriate for all fields
• Look at arbitrary-precision real-number libraries. Currently using Javascript's 64-bit floats, which are good up to at least 53-bit fixed-point numbers
• Test more edge cases

Basically I'm looking for feedback to make sure this is a good approach before polishing it all up. I could also use some direction on the aesthetic choices (does the current implementation look OK? How can we add more information to the table without cluttering it up?).