lsp 2

[andreasabel]

• From Agda 2.6.3 use Agda.TypeChecking.Error.explainWhyInScope
• Use showInfoError from EmacsTop rather than duplicating it here
• v0.2.6.4.0: Build with and embed Agda-2.6.4
• WIP: migrate to lsp-2 (does not build yet)

This PR is just to save the current progress, #22 should be merged first.
Currently need to migrate:

• Hover (lsp-2 dropped HoverContents and markedUpContent)
• Server.hs

[banacorn]

Thanks!