Removal of keyed property names

[matthewp]

This is a work in progress branch that removes keyed property names and adds:

• Additive multi-value properties.
• Named values for events like event: [one] click var(--onclick);, enabling multiple event listeners of the same type.
• New keywords unset and all. Note that these names will likely change before merging.

Closes #30

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 8ce9a2854aad72a5d0b131ec1de3cb2a79c29426

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------ | ----- | | corset | Major |

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