CppLanguageServer+LibGUI: Autocomplete #include paths

The C++ language-server can now autocomplete include paths.

Paths that start with '<' will be searched in /usr/include, and paths
that start with '"' will be searched in the project's root directory.

Userland/Libraries/LibGUI/AutocompleteProvider.h

@@ -23,6 +23,8 @@ public:
     enum class CompletionKind {
         Identifier,
         PreprocessorDefinition,
+        SystemInclude,
+        ProjectInclude,
     };
 
     enum class Language {