Support pinnable infoview messages

[rish987]

Similar to VSCode, it should be possible to pin/unpin infoview messages.

VSCode seems to support "live" messages that are automatically updated as the surrounding code changes. To me this seems a bit unexpected, and I would prefer to just have pinned messages be permanently "paused" and only update when specifically requested. Not to mention this would make the code simpler. Obviously if someone wants to simulate live updates this could be done with autocmds. Though keep in mind I don't have as much experience writing Lean code myself as reading it, so do let me know if there are other opinions.

[rish987]

Update: see neovim/neovim#15276 -- basically I think we should closely follow the way things are done at VSCode, which is to update pin positions on textDocument/didChange, rather than doing things ad-hoc with nvim_buf_attach() (the analogue of which was the case in VSCode before leanprover/vscode-lean4#30). This will guarantee that the call to infoview.__update() following updating the pin positions will come after actually having notified the server of the buffer changes.