Upstream Vala Language Server plugin

This is to upstream the plugin for the language server at https://github.com/Prince781/vala-language-server

See some release notes for a quick demonstration of features:

• https://github.com/Prince781/vala-language-server/releases/tag/0.48.3
• https://github.com/Prince781/vala-language-server/releases/tag/0.48.1
• https://github.com/Prince781/vala-language-server/releases/tag/0.48