LibJS: Add Javascript lexer and parser

This adds a basic Javascript lexer and parser. It can parse the
currently existing demo programs. More work needs to be done to
turn it into a complete parser than can parse arbitrary JS Code.

The lexer outputs tokens with preceeding whitespace and comments
in the trivia member. This should allow us to generate the exact
source code by concatenating the generated tokens.

The parser is written in a way that it always returns a complete
syntax tree. Error conditions are represented as nodes in the
tree. This simplifies the code and allows it to be used as an
early stage parser, e.g for parsing JS documents in an IDE while
editing the source code.:

Libraries/LibJS/Forward.h

@@ -30,6 +30,7 @@ namespace JS {
 
 class ASTNode;
 class Cell;
+class Expression;
 class Heap;
 class HeapBlock;
 class Interpreter;