Negative prop packs

`calc(-1 * var(--size-3))` is annoying to write and unintuitive to read. @felix-bohlin suggested in Discord to use `var(--size-3-)` with a trailing `-` to signify a negative value.

thoughts on this naming?
have alt naming suggestions?

These negative/inverted value props would be individual imports (aka not part of the main bundle) but would be included in the JS bundle for use with jit-props.

No file, test, or entry point is named. Start by reviewing the existing custom-property naming and import/bundle conventions, then resolve the proposed trailing-`-` syntax and its individual-import and JIT-bundle scope. Done means the naming decision and implementation scope are agreed.

Written by the indexing model from the issue text.