Terminal suggest: Support LSP-based completion provider

[Tyriar]

Prototype: https://github.com/microsoft/vscode/tree/tyriar/lsp_terminal_completions

cc @meganrogge

[Tyriar]

For this we need to be mindful of our fake resources showing up in other places like problems. We likely need a new scheme, and then we need to special case that scheme for things like not displaying problems. We do want LSP to evaluate problems, hovers, etc. as we may later use them, so no changes on the server side of things.