LibJS: Implement const variable declarations

This also tightens the means of redeclaration of a variable by proxy,
since we now have a way of knowing how a variable was initially
declared, we can check if it was declared using `let` or `const` and
not tolerate redeclaration like we did previously.

Libraries/LibJS/AST.h

@@ -470,6 +470,7 @@ private:
 enum class DeclarationType {
     Var,
     Let,
+    Const,
 };
 
 class VariableDeclaration : public Statement {