[api][visibility] Make the visibility macros more consistent

[api][visibility] Make the visibility macros more consistent

1. Follow the scheme introduced in PR #2501 for both `zdict.h` and `zstd_errors.h`.
2. If the `*_VISIBLE` macro isn't set, but the `*_VISIBILITY` macro is, use that.
   Also make this change for `zstd.h`, since we probably shouldn't have changed
   that macro name without backward compatibility in the first place.
3. Change all references to `*_VISIBILITY` to `*_VISIBLE`.

Fixes #3359.