And update the metaschema to accommodate the `isOverlay` properties
added in #8338. Overlay enums, like other overlay members, are
implemented out-of-band by the declaring package. Code generators should
not generate declarations for overlay enums.
Add a new `IsOverlay` option to schema types and functions that allows providers to document overlays in the schema. This makes it easier to generate API docs consistently, even for code that is generated outside of the typical codegen process.