Refactor DiagnosticProvider as an enum

This resolves a TODO in the core diagnostic module to refactor this
type. It was originally an alias of `LanguageServerId` for simplicity.
Refactoring as an enum is a necessary step towards introducing
"internal" diagnostics - diagnostics emitted by core features such as
a spell checker. Fully supporting this use-case will require further
larger changes to the diagnostic type, but the change to the provider
can be made first.

Note that `Copy` is not derived for `DiagnosticProvider` (as it was
previously because `LanguageServerId` is `Copy`). In the child commits
we will add the `identifier` used in LSP pull diagnostics which is a
string - not `Copy`.

helix-core/src/diagnostic.rs

@@ -50,8 +50,20 @@ pub struct Diagnostic {
     pub data: Option<serde_json::Value>,
 }
 
-// TODO turn this into an enum + feature flag when lsp becomes optional
-pub type DiagnosticProvider = LanguageServerId;
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub enum DiagnosticProvider {
+    Lsp { server_id: LanguageServerId },
+    // Future internal features can go here...
+}
+
+impl DiagnosticProvider {
+    pub fn language_server_id(&self) -> Option<LanguageServerId> {
+        match self {
+            Self::Lsp { server_id, .. } => Some(*server_id),
+            // _ => None,
+        }
+    }
+}
 
 // while I would prefer having this in helix-lsp that necessitates a bunch of
 // conversions I would rather not add. I think its fine since this just a very