LanguageServer: Handle late completion requests

Currently, Psalm Language Server is only able to handle completion
requests where the gap between a recent known type and the current
cursor position is exactly '->' or '::'. However, VS code often doesn't
send a completion request until an additional, third character has
been typed. This change allows Psalm to successfully serve
completion requests where an additional character has been typed.

src/Psalm/Codebase.php

@@ -1113,7 +1113,7 @@ class Codebase
             return null;
         }
 
-        $gap = substr($file_contents, $end_pos, $offset - $end_pos);
+        $gap = substr($file_contents, $end_pos, 2);
 
         return [$recent_type, $gap];
     }