This makes `0123` and `u0123` etc. illegal.

I'm now confident that this is a good change because
I actually caught two C header translation mistakes in `haiku.zig` with this.
Clearly, `0123` being octal in C (TIL) can cause confusion, and we make this easier to read by
requiring `0o` as the prefix and now also disallowing leading zeroes in integers.

For consistency and because it looks weird, we disallow it for integer types too (e.g. `u0123`).

Fixes #11963
Fixes #12417