Runtime: deliver runtime from css-blocks core?

This issue was originally filed by @amiller-gh against the runtime repo which has been merged into the css-blocks monorepo:

It doesn't make much sense to deliver as an independently versioned peer dependency. The rewriters will use a specific version of the runtime and that runtime will most likely be coupled to the version of css-blocks. Instead, css-blocks can:

1. Return to the rewriters the exact arguments that must be passed into the helper, instead of raw boolean expressions.
2. Expose the runtime in a well-defined location for rewriters to proxy up to the app they are integrated into.

This will reduce project module complexity and help to unify implementations of css-blocks across the board.

Start by mapping how the runtime and rewriters are currently packaged and connected in the css-blocks monorepo. Then assess the two proposed changes: returning helper arguments and exposing a runtime location for proxying. Done requires an agreed design and implementation scope, but the issue names no files or tests to begin with.

Written by the indexing model from the issue text.