transform assignment expressions into variable declarations if the variable is declared using a function argument

Example:
```js
function f(a, b, c) {
  console.log(b)
  b = 1;
  c = 2
}
```

Should produce:
```js
function f(a, b) {
  console.log(b)
  b = 1;
  var c = 2
}
```

I'll try to implement this myself c: