Fix to use more accurate types with custom isArray() and add lint (#5261)

* Fix to use more accurate types with custom isArray()

* Add lint against Array.isArray()

packages/codemirror-lsp-client/src/lib/utils.ts

@@ -0,0 +1,7 @@
+/**
+ * A safer type guard for arrays since the built-in Array.isArray() asserts `any[]`.
+ */
+export function isArray(val: any): val is unknown[] {
+  // eslint-disable-next-line no-restricted-syntax
+  return Array.isArray(val)
+}