lsp: disable warning notifications on server health warning by default

[mrcjkb]

@edevil since this PR, I sometimes get spammed with warnings like:

cargo check failed to start: Cargo watcher failed, the command produced no valid metadata (exit code: ExitStatus(unix_wait_status(25856))):
    Blocking waiting for file lock on package cache
    Updating crates.io index
error: failed to write /nix/store/5dh2vfghjizfwgci0z00hbrpx5vwv6zc-rust-lib-src/std/Cargo.lock

Caused by:
  failed to open: /nix/store/5dh2vfghjizfwgci0z00hbrpx5vwv6zc-rust-lib-src/std/Cargo.lock

Caused by:
  Read-only file system (os error 30)

Failed to read Cargo metadata with dependencies for `/nix/store/5dh2vfghjizfwgci0z00hbrpx5vwv6zc-rust-lib-src/std/Cargo.toml`: Failed to run `cd "/
nix/store/5dh2vfghjizfwgci0z00hbrpx5vwv6zc-rust-lib-src/std" && RUSTUP_TOOLCHAIN="/nix/store/p024jn1sd4pn33lg0kaqxh7j2sp1iw7x-rustc-1.80.1" "/nix/s
tore/c0r47wb8xm1dbsvppw6gh75jfjakal1y-cargo-1.80.1/bin/cargo" "metadata" "--format-version" "1" "--all-features" "--manifest-path" "/nix/store/5dh2
vfghjizfwgci0z00hbrpx5vwv6zc-rust-lib-src/std/Cargo.toml" "--filter-platform" "x86_64-unknown-linux-gnu"`: `cargo metadata` exited with an error:
   Updating crates.io index
error: failed to write /nix/store/5dh2vfghjizfwgci0z00hbrpx5vwv6zc-rust-lib-src/std/Cargo.lock

Caused by:
  failed to open: /nix/store/5dh2vfghjizfwgci0z00hbrpx5vwv6zc-rust-lib-src/std/Cargo.lock

Caused by:
  Read-only file system (os error 30)
Run ':RustLsp logFile' for details.
Press ENTER or type command to continue

I think we might need to disable warnings when the server health level is warning by default,
with an config to enable them.
This is far too noisy.

Originally posted by @mrcjkb in #509 (comment)

[edevil]

Yeah, that's probably Ok. TBH only the errors seem interesting so far.