[RFC] feat: Consistent `.$` in raw WGSL implemented functions

After layout, you need to add .$ before accessing bound resources. This does not work for slots:

  it('allows for re-resolves of slotted functions', () => {
    const slot = tgpu.slot<number>(1);
    const helper = tgpu.fn([], d.u32)`() { return slot.$ }`.$uses({ slot }).with(slot, 2);

    const main = tgpu.fn([])`() {
  let x = helper();
  let y = helper();
}`.$uses({ helper });

    expect(tgpu.resolve([main])).toMatchInlineSnapshot(`
      "fn helper() -> u32 { return 2.$ }

      fn main() {
        let x = helper();
        let y = helper();
      }"
    `);
  });

Also, we need docs for this.

Start with the inline test for re-resolves of slotted functions and trace tgpu.resolve together with .$uses({ slot }).with(slot, 2). Ensure the resolved helper emits 2.$ while calls from main remain valid, then add documentation covering the required .$ access after layout.