We must search the environment extension first, and then the builtin
table. Otherwise, the builtin declarations do not change when we
modify the files.
closes#1021
We need to report the jump target range in terms of a UTF-16 offset within the line, but to compute that we would have to load the target file, file-map it, and resolve that way each time the go-to-def handler runs. As an alternative, this stores the UTF-16 offsets in `DeclarationRange`.
