"Go To Type Definition" for type names should navigate to the type definition

"Go To Type Definition" doesn't doesn't work when the selection is itself a type. Instead, you get a "No type definition found" error. I can understand why it was implemented in this way ("types don't have a type!") but IMHO this behavior unnecessarily inconsistent and confusing for users. If I ask for the type definition of a type, then I should be taken to the... type definition! ;-) I'm filing this at @weswigham's request to split it out of #6209. IMHO, if #6209 is implemented as I'm suggesting (where Go To Definition would navigate to the actual implementing source, not the DefinitelyTyped .d.ts) then fixing this issue would be helpful because it'd make both operations more consistent: * Go To Definition would always navigate to implementation code (not to type declarations), regardless of what symbol is selected. * Go to Type Definition would always navigate to type declarations, regardless of what symbol is selected ## Search Terms Go to Type Definition No type definition found for `getTypeDefinitionsAtPosition` - this is one possible method to change in a PR ## Suggestion Go to Type Definition should, when called for a type, return the location of the type definition. ## Use Cases Discussed above ## Examples ```ts type X = {a: number}; const a: X = {a: 1}; ``` In VSCode, select and right-click on the second `X` and choose Go To Type Definition. Expected: navigates to declaration Actual: `No type definition for 'X'` ## Checklist My suggestion meets these guidelines: * [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code * [x] This wouldn't change the runtime behavior of existing JavaScript code * [x] This could be implemented without emitting different JS based on the types of the expressions * [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.) * [x] This feature would agree with the rest of [TypeScript's Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScr…