Add variables for global fontsizing
Add variables for global fontsizing

closes CNVS-25277

These will help our future-forward efforts for consistent font sizing.

Test Plan:
- This only adds variables that are hidden until used, none are used in
this commit
- No changes to ui
- CSS should compile successfully

Note:
- Based these on our most common sizes to date and what made the most
sense to label as xlarge, etc

Change-Id: I6e494589e63e92b15d63ff9d327ae3b0b3705e65
Reviewed-on: https://gerrit.instructure.com/69461
Reviewed-by: Jennifer Stern <7e594e5cba47371ac9ee694a84b9d94f9f6ec815@instructure.com>
Product-Review: Jennifer Stern <7e594e5cba47371ac9ee694a84b9d94f9f6ec815@instructure.com>
QA-Review: Colleen Palmer <387ba1943871e60965e6b44638a2f2e1ab9b8469@instructure.com>
Tested-by: Jenkins

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