[Splicing] Add `splicing` compile feature flag

[optout21]

Introduce a new splicing flag (hint: RUSTFLAGS="--cfg=splicing" ...).
The splicing flag should be independent of dual_funding (they can be used in isolation), but splicing implies the dual funding logic.
Dual funding parts should be surrounded with any(dual_funding, splicing), splicing-only with only splicing.
This is in preparation for splicing ( #1621 )
It is a rather mechanical infrastructure change but would make the future splicing PRs cleaner.

Prerequisites: #3137

Changes can be seen in #3274 , changes of the form #[cfg(...)].

[jkczyz]

Already added in #3001.