fix: use core font size setting vars

[anthonyronda]

Helps address #366 by using the following custom css properties defined in FVTT core. It does not only address chat cards! I'd like to address this as a serious accessibility concern and apply the changes across the system as much as possible.

--font-size-11: 0.6875rem;
--font-size-12: 0.75rem;
--font-size-13: 0.8125rem;
--font-size-14: 0.875rem;
--font-size-16: 1rem;
--font-size-18: 1.125rem;
--font-size-20: 1.25rem;
--font-size-24: 1.5rem;
--font-size-28: 1.75rem;
--font-size-32: 2rem;
--font-size-48: 3rem;
--line-height-12: 0.75rem;
--line-height-16: 1rem;
--line-height-20: 1.25rem;
--line-height-30: 1.875rem;

In some cases I was not able to match the existing font sizes and line heights exactly. In some cases I'm concerned that scaling will simply not work. I'm submitting as a draft so we can explore together what additional tweaks can be made

[anthonyronda]

Some example screen captures with the font size setting on a 7 (default is 5)

Some example screen captures with font size on a 10

Some of these could certainly be fixed with additional tweaking, but it will be tricky

[anthonyronda]

@wyrmisis I think I made a comment about font size in #479 (unless GitHub ate it)

I'd consider all font sizes should use these variables, because a user asked us for accessibility reasons to use Foundry's built in scaling