[Feature] Add support for generics

**Is your feature request related to a problem? Please describe.**
We've been implementing array sort function that requires generics to better implement the function's return type. Currently it's impossible to create a type of another argument's type.

**Describe the solution you'd like**
Introduce the following syntax:
```js
fun foo(arr: [T]) {
// We declare `other` with the same type as `arr`
let other = [T]
$ cmd that modifies other $
return other
}
```

**Additional context**
#966