LibJS: Implement "throw"

You can now throw an expression to the nearest catcher! :^)

To support throwing arbitrary values, I added an Exception class that
sits as a wrapper around whatever is thrown. In the future it will be
a logical place to store a call stack.

Libraries/LibJS/Lexer.cpp

@@ -62,6 +62,7 @@ Lexer::Lexer(StringView source)
         s_keywords.set("null", TokenType::NullLiteral);
         s_keywords.set("undefined", TokenType::UndefinedLiteral);
         s_keywords.set("return", TokenType::Return);
+        s_keywords.set("throw", TokenType::Throw);
         s_keywords.set("true", TokenType::BoolLiteral);
         s_keywords.set("try", TokenType::Try);
         s_keywords.set("typeof", TokenType::Typeof);