LibJS: Add spreading in array literals

Implement the syntax and behavor necessary to support array literals
such as [...[1, 2, 3]]. A type error is thrown if the target of the
spread operator does not evaluate to an array (though it should
eventually just check for an iterable).

Note that the spread token's name is TripleDot, since the '...' token is
used for two features: spread and rest. Calling it anything involving
'spread' or 'rest' would be a bit confusing.

Libraries/LibJS/Lexer.cpp

@@ -84,6 +84,7 @@ Lexer::Lexer(StringView source)
         s_three_char_tokens.set("<<=", TokenType::ShiftLeftEquals);
         s_three_char_tokens.set(">>=", TokenType::ShiftRightEquals);
         s_three_char_tokens.set(">>>", TokenType::UnsignedShiftRight);
+        s_three_char_tokens.set("...", TokenType::TripleDot);
     }
 
     if (s_two_char_tokens.is_empty()) {