feat(#3529): move letter spacing into seperate tokens

[Copilot]

This PR moves letter spacing values out of our heading tokens and into their separate values. It assumes the values were px and changes them to rem. These new tokens are implemented in GovAlta/ui-components#3606.

The tokens are the following:

--goa-letter-spacing-2xl:  -0.09375rem;   /* was -1.5  */
--goa-letter-spacing-xl:   -0.09375rem;   /* was -1.5  */
--goa-letter-spacing-l:    -0.078125rem;  /* was -1.25 */
--goa-letter-spacing-m:    -0.0625rem;    /* was -1    */
--goa-letter-spacing-s:    -0.046875rem;  /* was -0.75 */
--goa-letter-spacing-xs:   -0.046875rem;  /* was -0.75 */
--goa-letter-spacing-2xs:  -0.03125rem;   /* was -0.5  */

Original prompt

Address this issue:
GovAlta/ui-components#3529

Move the letter spacing out of the global typography values into separate values.

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[willcodeforcoffee]

LGTM

[chrisolsen]

@bdfranck there are conflicts that need to be resolved

[bdfranck]

I've resolved the merge conflicts and fixed the commit message. 👍