AK: Move FormatParser definition from header to implementation file

This is primarily to be able to remove the GenericLexer include out of
Format.h as well. A subsequent commit will add AK::Result to
GenericLexer, which will cause naming conflicts with other structures
named Result. This can be avoided (for now) by preventing nearly every
file in the system from implicitly including GenericLexer.

Other changes in this commit are to add the GenericLexer include to
files where it is missing.

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/GenericLexer.h>
 #include <AK/Singleton.h>
 #include <AK/StringBuilder.h>
 #include <Kernel/Debug.h>