add type hierarchy provider

[Eskibear]

This PR is part of Type Hierarchy API.
corresponding LSP proposal in microsoft/language-server-protocol#1231
early investigation here: microsoft/vscode-languageserver-node#779

This PR includes:

• type hierarchy provider and registry
• related API in vscode.proposed.d.ts

[Eskibear]

@dbaeumer @jrieken
So far I only tested the proposed api with a sample extension, and the provider was successfully registered.
Please help to keep me on the right track. Thanks.

[jrieken]

Some nit, enforcing proposed API is the most important

[jrieken]

Thanks, this is looking pretty good already

[jrieken]

Thanks @Eskibear

[jrieken]

fyi - I have pushed f50840e which adds an API command test.

vscode/src/vs/workbench/test/browser/api/extHostApiCommands.test.ts

Line 1340 in f50840e

test('TypeHierarchy, back and forth', async function () {

There was actually a bug in call hierarchy that also made it into the type hierarchy. This should be resolved now which is goodness. Using the call hierarchy in the references viewlet should now be possible