Consolidate check output to single Diagnostic type

[emdoyle]

This PR is a refactor with the goal of consolidating the idea of a 'Diagnostic' emitted by Tach.

This results in simpler code in many places, and better matches the mental model of Tach as a linter at its core.
For example:

• the LSP server can easily map our Diagnostic type to lsp_types::Diagnostic, we immediately solve Publish warning diagnostics from tach server #490 without separate code for warnings
• checks related to ignore directives produce Diagnostics, and can be plugged into check, check-external without changes
• formatting logic can be consolidated into one place, and handle Diagnostics produced by any check (this also solves Segment dependency errors and deprecation warnings in tach check output #485 )