Deprecate implicit conversion between signed and unsigned integers

[Walter Bright]

On 2/6/2025 7:18 AM, Quirin Schroll wrote:
> 4. Micro-lossy narrowing conversions:
>      * `int`/`uint` → `float`
>      * `long`/`ulong` → `float`/`double`

We already do VRP checks for cases:

```
float f = 1; // passes
float g = 0x1234_5678; // fails
```