The contract between Dune CMS and its plugins, as types.
A leaf package: no dependency on @dune/core or @dune/theme-api. Core imports
it and type-tests that what it hands a plugin is assignable to these
declarations, and that what a plugin hands in is accepted by core.
Born at 1.0.0 (no release candidate) together with Dune 0.36. See CHANGELOG.md
for what it declares.