[On-Demand Definitions] Initial draft (#67)

* Adds initial draft of the static `define` proposal.

* fixup! Renames proposal to "On-Demand Definitions"

* fixup! Updates HydroActive discussion with new API and usage of this proposal.

* fixup! Adds some sections based on initial feedback and updates example implementation to match.

* fixup! Updates template status to "Proposal" as that is the first stage of a community protocol.

* fixup! Adds sections on development-only checks, an alternative convention which inlines the `define` implementation at its usage sites, and an alternative using `customElements.whenDefined`.

Also makes various minor readability improvements and acknowledges the new scoped custom element registries proposal.

* fixup! Revert edits to `proposal-template.md`, which was separated into a distinct PR.