[proposal] Persistent variables in functions/methods

[Neek Sandhu]

It'd be really useful to have variables inside methods/functions that are initialized once, reused for subsequent calls and live as long as containing scope i.e the function itself.

> not to be confused with `static` properties

## Use Cases

Almost every app has a function or method that needs to "preserve" some state across multiple calls, a counter for example. Current ways out of this situation are either closures created with IIFE's or making those variables top-level. Both of which are ugly. I think the same could be done much more neatly with persistent variables.

## Example

```javascript
function foo(n) {
    // initialized on first call to foo, persists for subsequent calls
    persist let counter =  0;

    // this way JS engine won't have to recompile the pattern everytime
    persist const httpRE = /^https?/;

    counter++;
    return n * a;
}
```

is 1:1 as

```javascript
let foo = (() => {
    let counter = 0;
    const httpRE = /^https?/;
    return (n) => {
        counter++;
        return  n * a;
    }
})()
```

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