ggml : update ggml_prec specification (#26675)
* ggml : update ggml_prec specification
[no ci]
* cont : add GGML_PREC_BF16
* cont : rework API
* cont : use new API
* cont : swap arg order
* cont : support for MUL_MAT_ID
* cont : fix accidental remove of "break;"
* cont : return bools, add doc TAG_GGML_PREC, clean-up
* cont : add search tag
* cont : ws
