impr: Unify `TgpuMutable` and `TgpuBufferMutable` (and the `Readonly` versions)

Intuitively calling:
```ts
const countMutable = root.createBuffer(d.u32).$usage('storage').as('mutable');
```
should be equivalent to calling:
```ts
const countMutable = root.createMutable(d.u32);
```
But the types are entirely different