Add `readonly` and `const` Parsing in `ts2mls`

[NeilKleistGao]

Related Issue: #134

[NeilKleistGao]

I wonder if types like mut string acceptable. I made some changes in NewParser.scala because I found that we can do like this in OCaml:

val x : int ref = {contents = 42}

[NeilKleistGao]

We will continue to work on this PR after this syntax's implementation:

class Foo { x: T; mut y: S; ... }