Migrate to pnpm

Migrate from yarn to pnpm per Diamond's UI policy

Author: callumforrester

.github/workflows/build.yml

@@ -23,5 +23,5 @@ jobs:
       with:
         node-version: ${{ matrix.node-version }}
         cache: 'npm'
-    - run: yarn install
-    - run: yarn build
+    - run: pnpm install
+    - run: pnpm build

.github/workflows/lint.yml

@@ -22,5 +22,5 @@ jobs:
       with:
         node-version: ${{ matrix.node-version }}
         cache: 'npm'
-    - run: yarn install
-    - run: yarn lint
+    - run: pnpm install
+    - run: pnpm lint