This adds a new set of design tokens for working with text colors. There

are now explicit on-light/dark text color tokens, with weak variants.

These are automatically used by text-color and text-color-weak based

on the Ink color scheme.
surface-fg and surface-fg-weak now map to text-color and

text-color-weak, making it easier to change the appropriate text color

across the board.
A new ink-g-shadow-l-dark token helps with making larger, darker

shadows, used for things like modal dialogs.
An ink-i-mobile-menu token was added for use in hamburger menus on

mobile devices.

Made use of these in some in-progress changes for Review BOard. Verified

the appearance in light and dark modes.