Start diagnostic group_id at 1 to handle non LS diagnostics (#22694)

In particular, `DiagnosticPopover` both:

* Supports moving the selection to a diagnostic when clicked, based on
`group_id`

* Provides Diagnostic values with `group_id: 0` providing informztion on
hover about invisible characters.

So, clicking such a popover would navigate to the very first error
produced by a language server. Really not a big deal of course, but
seems good to fix as it might result in surprising behavior in other
future circumstances

Release Notes:

- N/A

crates/language/src/buffer.rs

@@ -208,10 +208,10 @@ pub struct Diagnostic {
     /// The human-readable message associated with this diagnostic.
     pub message: String,
     /// An id that identifies the group to which this diagnostic belongs.
+    /// 0 is used for diagnostics that do not come from a language server.
     ///
-    /// When a language server produces a diagnostic with
-    /// one or more associated diagnostics, those diagnostics are all
-    /// assigned a single group ID.
+    /// When a language server produces a diagnostic with one or more associated diagnostics, those
+    /// diagnostics are all assigned a single group ID.
     pub group_id: usize,
     /// Whether this diagnostic is the primary diagnostic for its group.
     ///