refactor: switch from npm to pnpm

This commit also installs the terser-webpack-plugin, since it was
implicitly installed by npm but not by pnpm, and we need it.

Fixed: twpowertools:211
Change-Id: Ie1f07c8fb8d089d70f56c058dbd6b4a8c44eb3c4
diff --git a/roles/build-extension/tasks/main.yaml b/roles/build-extension/tasks/main.yaml
index 6ca0225..be6b5ce 100644
--- a/roles/build-extension/tasks/main.yaml
+++ b/roles/build-extension/tasks/main.yaml
@@ -1,3 +1,11 @@
+- name: Install pnpm
+  ansible.builtin.command: npm install -g pnpm
+  args:
+    chdir: "{{ zuul.project.src_dir }}"
+
+- name: Ouptut pnpm version
+  ansible.builtin.command: pnpm --version
+
 - name: Build extension
   make:
     chdir: "{{ zuul.project.src_dir }}"