Issue Description:

Implement the textDocument/implementation request, which given a source location, resolves the symbol at that location, and returns the implementations of a given protocol, or protocol requirement.

https://microsoft.github.io/language-server-protocol/specification#textDocument_implementation

To implement this, you can start by looking at how jump to definition (textDocument/definition) resolves the symbol under the cursor, but instead of looking for its definition, look for all symbols that have a base relation with it (to find types that implement a protocol) or override relation (to find methods/properties that implement a protocol requirement). All of the database query methods to do this should already exist in indexstore-db's C++ code, although you may need to expose them in the Swift wrapper.