Move \RemoveEmptyStaves to new file for context modifications.
This allows \RemoveEmptyStaves to be invoked outside \layout blocks, i.e.,
directly following a context instantiation:
\new Staff \RemoveEmptyStaves { ... }
or
\new Staff \with { \RemoveEmptyStaves } { ... }
* input/regression/remove-empty-context-mod.ly
new regtest
* ly/context-mods-init.ly:
new file for context modification identifiers
* ly/declarations-init.ly:
include context-mods-init.ly; placed before engraver-init.ly to ensure
\RemoveEmptyStaves is accessible for deprecated \RemoveEmptyStaffContext
and analogues