Add support for GVls
Prerequisites
- [x] I have searched open and closed issues for duplicates.
Feature
Code requires a completion engine and more features provided by a Language Server Protocol.
See GVls for Code in action:

Describe the solution you'd like Add support for GVls to provide such services.
Existing work I've created the https://github.com/elementary/code/pull/599 to introduce GVls to, but it is blocked due to lack of Debian packages, so this issue is to request to add GVls as a subproject.
Describe alternatives you've considered There other language servers out there, some of them has been abandoned, other just don't find the way to integrate on IED.
GNOME Builder has a language server, but it is not reusable by Code.
Additional context Adding completion will push Vala coding productivity and potencially.