refactor(qa): split Matrix QA into optional plugin (#66723)

Merged via squash.

Prepared head SHA: 27241bd0898d9ed1f13a5aaf3113be345d3002be
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

test/scripts/bundled-plugin-build-entries.test.ts

@@ -82,6 +82,9 @@ describe("bundled plugin build entries", () => {
     expect(artifacts.some((artifact) => artifact.startsWith("dist/extensions/qa-lab/"))).toBe(
       false,
     );
+    expect(artifacts.some((artifact) => artifact.startsWith("dist/extensions/qa-matrix/"))).toBe(
+      false,
+    );
   });
 
   it("keeps bundled channel secret contracts on packed top-level sidecars", () => {