Mutable top-level variables become immutable

[Booksbaum]

Mutable variables in modules and namespaces (#393) were fixed in #398, but for top-level variables that issue still remains:

export var v: number;
export let l: number;

==>

let [<Import("v","...")>] v: float = jsNative
let [<Import("l","...")>] l: float = jsNative

It should be

let [<Import("v","...")>] mutable v: float = jsNative
let [<Import("l","...")>] mutable l: float = jsNative

BUT: ts2fable places everything in a recursive module:

module rec Name
open System
//...
let [<Import(...)>] mutable v: float = jsNative

In a rec module, mutable let bindings aren't allowed:

error FS0874: Mutable 'let' bindings can't be recursive or defined in recursive modules or namespaces

-> requires a bigger rewrite than #393/#398 -- and a bigger change of generated code for top-level variables.

(extra issue for better tracking -- and #393 doesn't mention top-level variables (only tried while testing the fix))