Exposes the registerDiagram() function publically as
`mermaid.mermaidAPI.registerDiagram` so that users can add their
own diagrams at bundle-time.
This is instead of using the lazyLoadedDiagrams config setting.
Removes vitest from all subpackages so that vitest/`pnpm run test`
are only in the root mono-repo.
This is required, because otherwise the root vitest and the subpackage
vitest versions can otherwise be slightly different, which causes
issues when running unit tests:
```Snapshot cannot be used outside of test```
In the future, we may want to consider moving package specific
tests into `packages/*/test`, and instead running these tests
with `pnpm run --recursive`, so that tests are run in each package by
their own version of vitest. This is the way that most projects
do things (e.g. https://github.com/vitest-dev/vitest).
