LanguageServers/Cpp: Add 'FindDeclaration' capability

The C++ LanguageServer can now find the matching declaration for
variable names, function calls, struct/class types and properties.

When clicking on one of the above with Ctrl pressed, HackStudio will
ask the language server to find a matching declaration, and navigate
to the result in the Editor. :^)

Userland/DevTools/HackStudio/LanguageServers/LanguageClient.ipc

@@ -1,4 +1,5 @@
 endpoint LanguageClient = 8002
 {
     AutoCompleteSuggestions(Vector<GUI::AutocompleteProvider::Entry> suggestions) =|
+    DeclarationLocation(String file_name, i32 line, i32 column) =|
 }