[lldb-vscode] support the completion request

Summary:
The DAP has a completion request that has been unimplemented. It allows showing autocompletion tokens inside the Debug Console.
I implemented it in a very simple fashion mimicking what the user would see when autocompleting an expression inside the CLI.
There are two cases: normal variables and commands. The latter occurs when a text is prepepended with ` in the Debug Console.
These two cases work well and have tests.

Reviewers: clayborg, aadsm

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69873