Epic: Use CSS Variables in Grafana Theme

We want to use CSS variables in the grafana theme for performance benefits. @ashharrison90's 'ideal future': - everyone still uses `useTheme2`. we have some good lint rules built around it, it ensure consistent syntax, the intellisense is nice, etc. - the "theme" exposed by `useTheme2` follows the same scaffolding, but instead of hardcoded color tokens, all the values are css variable references, e.g. ``` theme = { colors: { text: { primary: "var(--gf-theme-colors-text-primary)", ...etc }, ...etc }, ...etc } ``` - changing the theme doesn't change the value of theme being used by `useTheme2`/ThemeContextProvider, that will always use this "variable theme" - instead, changing the theme changes the values of the css variables on the root element.