Promote `ZSTD_c_targetCBlockSize` Parameter to Stable API

Promote `ZSTD_c_targetCBlockSize` Parameter to Stable API

This feature has demonstrated itself to be useful in web compression and we
want to encourage other folks to use it. But we currently make it difficult
to do so since it's locked away in the experimental API.

The API itself is really straightforward and I think it's fine to commit to
maintaining support / compatibility for this API even if in the future the
underlying implementation may continue to evolve.

Note that this commit changes its enum name and also its numeric value. Users
who respected the instructions of using the experimental API should be fine
with both of these changes since they should only have referred to it by the.

Conceivably someone could have done bad feature detection of this capability
by doing `#ifdef ZSTD_c_targetCBlockSize` which will now return false since
it's no longer a macro... but I think that's an acceptable hypothetical
breakage.