src: make `FIXED_ONE_BYTE_STRING` an inline fn

This prevents accidental usage on non-fixed strings.

PR-URL: https://github.com/nodejs/node/pull/22725


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>