Suppress warning notifications from LSP servers and widget API proposal

[harry-xm]

Background

Open a Rust file in https://github.com/eunomia-bpf/eunomia-bpf/tree/master/ecli on macOS and LSP4IJ turns every warning message from rust-analyzer into a notification that is persisted in the Notifications panel.

Plan

In order to turn it into a status bar widget that reflects warnings like the rust-analyzer VSCode plugin, the following APIs are needed to implement it in a language plugin.

1. A status bar widget API, (optionally) with which plugins can customize its normal, warning and error style. If clicked, it should open a panel that lists warnings and errors from the LSP server.
2. API to suppress warnings from LSP servers.

Each file can only have one active LSP server, so it makes sense to provide a unified experience from LSP4IJ. If the widget API is too heavy for LSP4IJ, API to get and register for notifications of LSP servers' status and lifecycle changes is an alternative that allows plugins to manage the widgets themselves.

If LSP4IJ could not provide a warnings and errors panel for LSP servers, API to fetch or collect warning and error messages from LSP servers is needed.

Originally posted by @harry-xm in #493