Extract the type definitions for the `<mosaic-book>` element provided by
VitalSource into `src/types/vitalsource.ts` and use JSDoc + TS to
typecheck the fake implementation in vitalsource-mosaic-book-element.js.

Extracting the definitions into a standalone module avoids the need for TS to
process all of the transitive dependencies of src/annotator/integrations/vitalsource.ts
when processing the dev-server/tsconfig.json project.