Parses Priority header field value pointed by *value* of length
*len*, and stores the result in the object pointed by *extpri*.
Priority header field is defined in :rfc:`9218`.

This function does not initialize the object pointed by *extpri*
before storing the result.  It only assigns the values that the
parser correctly extracted to fields.

This function returns 0 if it succeeds, or one of the following
negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
    Failed to parse the header field value.