LibCore: Add File method to determine absolute path

This will generate absolute paths lexically rather than through a call
to realpath. The motivation for this is to generate absolute paths for
non-existent files in unveil calls, as realpath will not work if the
file does not exist.

Userland/Libraries/LibCore/File.h

@@ -36,6 +36,7 @@ public:
     static bool exists(const String& filename);
     static bool ensure_parent_directories(const String& path);
     static String current_working_directory();
+    static String absolute_path(String const& path);
 
     enum class RecursionMode {
         Allowed,