`source.request.activeregions` is a new request that reports all
`#if`, `#else` and `#elseif` decls in the response with an `is_active` flag.
This is mainly useful for a client to know which branches are active,
but for completeness sake, the active decls are also reported.
Note: it only reports the positions of the decls, not the entire ranges.
It's up to clients to map this to the syntactic structure of a tree.
Reporting the ranges of the decls could be confusing in the case
of nested `#if`s, where the ranges can overlap.
