LibSQL: Add Parser::consume_if helper

The following is a common (and soon to be *very* common) expression:

    if (match(token_type))
        consume();

Using consume_if() makes this a bit simpler and makes it less likely to
forget to invoke consume() after the match().

Userland/Libraries/LibSQL/Parser.h

@@ -75,6 +75,7 @@ private:
 
     Token consume();
     Token consume(TokenType type);
+    bool consume_if(TokenType type);
     bool match(TokenType type) const;
 
     void expected(StringView what);