Replace explicit tracking scale with multiplier system

[aaronadamsCA]

tracking-tight can also be expressed as -tracking-wide, which seems like an API design smell.

I also increasingly find myself needing arbitrary values like tracking-[0.0125rem].

For both these reasons, I'd like to suggest a multiplier system like #14857 for tracking:

• The theme defines only the tracking scale --tracking: 0.025rem.
• A migration replaces named values with scale values:
  • tracking-tighter to -tracking-2
  • tracking-tight to -tracking-1
  • tracking-normal to tracking-0
  • tracking-wide to tracking-1
  • tracking-wider to tracking-2
  • tracking-widest to tracking-4
• Named values probably get deprecated static utilities to ease migration.

This is simple and intuitive, fixes the design smell, and supports in-between values like tracking-0.5.

Tracking isn't like other named scales (like text size, leading, and radius), because it (a) supports negative values and (b) doesn't really benefit from a constrained set of named values. In those two respects it's a lot more like spacing, so I think it's a great candidate for a spacing-like approach.