Switch from yarn to pnpm

We've started using pnpm throughout all Happo repos, and we find that
it's easier to use and faster.

Author: trotzig

.circleci/config.yml

@@ -9,5 +9,13 @@ jobs:
       - checkout:
           method: blobless
 
-      - run: yarn install
-      - run: yarn test
+      - run:
+          name: Install pnpm package manager
+          command: |
+            npm install --global corepack@latest
+            corepack enable
+            corepack prepare pnpm@latest-10 --activate
+      - run:
+          name: Install Dependencies
+          command: pnpm install
+      - run: pnpm test