keep types.d.ts always up-to-date with Vaadin version (#442)

When a `types.d.ts` file is missing, vaadin-maven-plugin automatically generates an up-to-date default version of it on the first build (when running `mvn`).

Keeping a pre-generated file in this repo means it needs to be updated with every Vaadin version that changes its contents. This one extra maintenance task was forgotten in the V18 release, which lead to issues using the [feature of importing CSS directly as LitElement styles](vaadin/flow#8843) in projects cloned from this repo (i.e. all start.vaadin.com projects).