[2025-02 CWG Motion 9] P1967R14 #embed

Redistributed grammar around the preamble to better
maintain a locality and flow.  More redistrubution
expected in a follow-up commit.

Minor grammar tweak: "other than a" -> "except:"
following the model of such specifications in
basic-s-char and elsewhere.

Attempted to describe all implementation-defined
topics for the index, but will need review.

Author: AlisdairM