Migrate to pnpm

Migrate from yarn to pnpm per Diamond's UI policy

Author: callumforrester

.github/workflows/build.yml

@@ -22,6 +22,6 @@ jobs:
       uses: actions/setup-node@v3
       with:
         node-version: ${{ matrix.node-version }}
-        cache: 'npm'
-    - run: yarn install
-    - run: yarn build
+        cache: 'pnpm'
+    - run: pnpm install
+    - run: pnpm build

.github/workflows/lint.yml

@@ -21,6 +21,6 @@ jobs:
       uses: actions/setup-node@v3
       with:
         node-version: ${{ matrix.node-version }}
-        cache: 'npm'
-    - run: yarn install
-    - run: yarn lint
+        cache: 'pnpm'
+    - run: pnpm install
+    - run: pnpm lint