Add type hierarchy provider as Language Server capability

[lars-reimann]

Closes #1275

• Handle type hierarchy requests in the Language Server
• Add TypeHierarchyProvider as a new LSP service
• Add the interface TypeHierarchyProvider and a no-op abstract implementation AbstractTypeHierarchyProvider to easily customize the responses for a given language
• Add a type hierarchy provider for interfaces in Langium grammars

Test instructions:

Some automated tests are included. To check functionality in VS Code, create a Langium grammar with the following content:

interface A {}
interface B extends A {}

Place the cursor on the name of either interface and invoke the action "Types: Show Type Hierarchy". Via the button you can switch between showing subtypes and supertypes of the selected interface.

[lars-reimann]

Thanks for the feedback. I've adjusted the PR accordingly.