Refactor EditorUIService to use LSP showMessage and showMessageRequest

[JustinGrote]

Prerequisites

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Summary

EditorUIService currently uses custom messages to handle prompts. They should use the LSP spec for this instead so it doesn't have to be handled special in the client.

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#window_showMessageRequest

Proposed Design

Replace existing messaging with LSP but keep methods the same if possible to avoid breaking.