Add support for local variables

[tronical]

Discussed in https://github.com/slint-ui/slint/discussions/2751

^{Originally posted by **loloof64** May 18, 2023} Hi, is it possible to define local variables in ui scripts components/functions ? I mean something like this for example ```javascript export component MySquare inherits Rectangle { width: 100px; height: 100px; background: red; in property items: "abcd/efgh/ijkl/mnop"; public function get_items_arr() -> [string] { let arr = []; for elt in items { if (elt != "/") arr.push(elt); } arr } } ```

[ogoffart]

Local variables might make some sense. But i would be hesitant to implement for loop. Our strategy had been to keep the language intentionally simple because the actual logic should be written in native code.

[ogoffart]

We would need to decide a syntax for it.

I'd go with let as this is the same syntax as rust and typescript.

let foo : string = "123";

(type annotation optional)

[Tmpod]

I agree with this. Having variables is useful sometimes, but Slint shouldn't really go deeper than that, and let the backend language handle that.

[hunger]

Maybe variable <string> foo: "bar"; ? That would align with the property syntax a bit more closely.

Or var <string> foo: "bar"; ?

[FloVanGH]

The code for the timer picker #5087 would also profit from local variables.