framework: add vala.mk -- easily depend on lang/vala

The vala library comes with the version in the name.
Instead of updating the LIB_DEPENDS on every major vala update,

LIB_DEPENDS=libvala-XX.so:lang/vala

one can now just have a static

USES=vala:lib

For build-time dependencies on vala or valac, use

USES=vala:build

PR: 249940