Theme: Use unitless values for typography line-height tokens

[aduth]

Follow-up to #73931 (comment)

Related: #64340

What problem does this address?

The current set of typography tokens don't provide much flexibility in mixing and matching line heights and font-sizes, as the current set of line heights uses fixed pixel values rather than relative units (either unitless or relative units like rem/em).

What is your proposed solution?

Using unitless values for line-heights would enable greater flexibility in how those line-heights can be combined with different font sizes.

Needs Design Feedback:

• This proposal contradicts earlier proposals in Systematising typography styles in the admin UI #64340 to use pixel values to ensure alignment using multiples of 4px. Is there a compromise solution which achieves both goals?
• If we use unitless or relative values, what are those values?
• Is this based on a faulty premise of mixing and matching font sizes and line heights, where they should instead be kept in sync (i.e. only using md line-height with md font size).