Implement TypeDefinitionProvider

[olafurpg]

Monaco 0.9.0 (merged in #49) added a new TypeDefinitionProvider service
https://microsoft.github.io/monaco-editor/api/interfaces/monaco.languages.typedefinitionprovider.html

It would be nice to implement it in metadoc. The service provides a way to jump to the type definition when hovering over a non-type symbol (e.g., in term or pat position).
Example motivation microsoft/TypeScript#10394

olafurpg@c17d2d9 shows a skeleton of how to implement this service in metadoc.