Language server support

Currently there is limited coding / scripting support in code editors like vscode and others. The current support is limited to syntax highlighting via an extension.

https://github.com/d5/tengo/issues/146

Most modern languages implement a language server via language server protocol which enables code editors to receive information about the code like symbols, help text, code completion suggestions, snippets,... An implementation via language server protocol would cover most of the used code editors (vs code, atom, goland).

Because the tengo language is not a completely new language I think that parts of current implementation could be reused, namely parts of the golang language server.

Further code assistance feature could also allow the language to gather more users and so maybe also more contributors.