Adds different font-sizes and line-heights for the custom
font-size UI. The custom font-size preference options are now:
1. Compact - 14px font-size & 22px line-height
2. Standard - 16px font-size & 24px line-height
3. Open - 20px font-size & 26px line-height
This removes the "xl" font-size option
(`vector-feature-custom-font-size-3`) and it's related i18n messages,
and changes the remaining message to match the new options.
The default font-size, when the custom font-size feature is disabled,
is "compact".
Bug: T351693
Change-Id: I5975a78ce7327f314fd55de55c222ac08d801fbd
Introduces CSS custom properties for the purposes
of enabling client-side customizable font sizes.
CSS custom properties are contained in a Less file,
which imports values from Codex and converted them to rems
using the Less unit() function.
Includes ADR on CSS custom properties decisions.
Bug: T348984
Change-Id: Ic5e62d55f3c3585fe3a00e41f29f88d3a2f7470b
