feat: add validation for letter spacing that allows negative values

[mxschll]

Issue #, if available:

Description of changes:

We want to make our heading letter-spacing tokens public and themable. However, because these tokens can contain negative values (which are valid), the current validation fails, preventing us from making them public. This PR separates letter-spacing validation from other font-sizing logic.

With this change, css keyword values are possible and numeric values in the following format:

'1px', '-0.5rem', '0em', '0.25px', '-1.5em', '.5px', '-.5rem'

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.51%. Comparing base (b545df9) to head (665a8f1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #138   +/-   ##
=======================================
  Coverage   97.51%   97.51%           
=======================================
  Files          48       48           
  Lines        2612     2616    +4     
  Branches      508      540   +32     
=======================================
+ Hits         2547     2551    +4     
  Misses         65       65           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[amanabiy]

This pattern match partial strings like revert-something or 12px extra-text. Should we use anchors and word boundaries to ensure only exact matches are accepted something like this: ^(\b(normal|inherit|initial|revert-layer|revert|unset)\b|-?\d*\.?\d+(px|rem|em))$.

I see the others also have similar implementation so I am wondering if we can do this maybe as a follow up.