feat: add in-memory diagnostics collector

Author: tharropoulos

src/diagnostics.ts

@@ -0,0 +1,13 @@
+import type { ApiDiagnostic, DiagnosticCollector } from "./types.js";
+
+export class MemoryDiagnosticCollector implements DiagnosticCollector {
+  readonly #items: ApiDiagnostic[] = [];
+
+  add(diagnostic: ApiDiagnostic): void {
+    this.#items.push(diagnostic);
+  }
+
+  snapshot(): readonly ApiDiagnostic[] {
+    return [...this.#items];
+  }
+}