improve tools for rules debugging

[bam80]

Continuation of https://gitlab.freedesktop.org/xkeyboard-config/xkeyboard-config/-/merge_requests/793#note_2797505

Q: Any tool showing resulting symbols string, to debug rules?

A: There is unfortunately no public API for this. Either:

• use the --verbose options or compile libxkbcommon
• compile libxkbcommon locally, then use the internal tool build/compile-keymap --kccsgt --include … --layout … --variant ….

So it's buried really deep for now, and is not documented.
Even if someone is lucky enough to deduce it, he has to compose complex pipes currently:

$ xkbcli compile-keymap --verbose --test --model applealu_iso --layout us  2>&1 | grep KcCGST

Could we improve the situation e.g. by adding xkbcli compile-keymap --kccsgt option, or expanding existing --rmlvo option there?