This introduces the beginnings of the LessCSS codebase, in preparation

for the default theme and the first components.
Ink's LessCSS code is based on our CardStock experiments, which builds

components in three layers:


Schema layer: Schemas are LessCSS mixins that define the

   structure of a component, but not the rules.
They provide all the CSS selectors that comprise a component and

 offer named rulesets that the caller can provide in order to fill out

 those selectors.
Schemas are used to populate rules for a component and also to

 specialize rules for a component without being affected when any part

 of the underlying structure changes.


Component layer: These are mixins that define the components'

   basic styles and layout.
They take in a name for the component and some other state and then

 make use of a schema to fill out the styling.
The component layer itself does not output CSS. Instead, its mixins

 can be invoked to output CSS with a specified component name and any

 relevant arguments. This allows a component to specialized under

 another name, which is useful if one needs a very custom variant of a

 button but doesn't want to copy/paste a bunch of styling or mix in

 the original classes.


Theme layer: The theme injects the CSS for the components, using

   the component mixins, and adds any CSS variables that fulfill the

   contract of the component.
By injecting at this layer, we minimize the amount of CSS that

 ultimately needs to be generated, helping keep the resulting

 stylesheet tidy and making it easier to specialize aspects of it.
It's also responsible for defining any units, palettes, and other

 design tokens that are used by components.


This change contains the support for defining schemas and components,

and the base support for defining themes, but otherwise results in an

empty stylesheet. Upcoming changes will introduce actual styles.
The base theme support does contain some very useful CSS variables that

act as feature flags. These include --if-dark, --if-dark-only,

--if-light, and --if-high-contrast, and are used like:

property: var(--if-dark, dark-value)
          var(--if-light, light-value)
          var(--if-high-contrast, high-contrast-value);


Depending on the theme used (system or an explicit light, dark, or

high-contrast), the appropriate flag will fall back to the value that

accompanies the flag, and the others will convert to empty strings. This

is an effective way of defining switches for styles, and will be used

throughout the upcoming default theme.

Been working extensively with this foundation to draft a half dozen

components and a base theme we can use in Review Board. All provided

functionality has been used.