LibJS: Forward-declare RegexTable and BasicBlock in Executable.h

Previously every file that included Executable.h (which is pretty much
most LibJS and LibHTML files, given that VM.h needs it) had the whole
definition of LibRegex, which was slowing down source parsing.

Userland/Libraries/LibJS/Bytecode/Executable.cpp

@@ -4,7 +4,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibJS/Bytecode/BasicBlock.h>
 #include <LibJS/Bytecode/Executable.h>
+#include <LibJS/Bytecode/RegexTable.h>
 #include <LibJS/SourceCode.h>
 
 namespace JS::Bytecode {