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/LanguageServer.ipc

@@ -9,4 +9,6 @@ endpoint LanguageServer = 8001
 
     AutoCompleteSuggestions(String file_name, i32 cursor_line, i32 cursor_column) =|
     SetAutoCompleteMode(String mode) =|
+    FindDeclaration(String file_name, i32 line, i32 column) =|
+
 }