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/Token.h

@@ -105,6 +105,7 @@ enum class TokenType {
     Slash,
     SlashEquals,
     StringLiteral,
+    Throw,
     Tilde,
     Try,
     Typeof,