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](https://gitlab.gnome.org/esodan/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.