revert postcss-svgo to 4.x

<input css uSVb3S>
71:37	⚠  Error: Plugin name should be specified [postcss-svgo]
73:36	⚠  Error: Plugin name should be specified [postcss-svgo]
158:35	⚠  Error: Plugin name should be specified [postcss-svgo]

.github/workflows/ci.yml

@@ -63,3 +63,40 @@ jobs:
       with:
         name: lhci-results
         path: ./.lighthouseci
+
+  lint:
+    runs-on: ubuntu-latest
+    if: "!contains(github.event.head_commit.message, '[skip ci]')"
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 1
+        lfs: false
+    - uses: actions/setup-node@v2
+      with:
+        node-version: 14.x
+    - name: Get Yarn cache path
+      # https://github.com/actions/cache/blob/master/examples.md#node---yarn
+      # https://github.com/actions/cache/issues/60
+      id: yarn-cache
+      run: echo "::set-output name=dir::$(yarn cache dir)"
+    - name: Cache dependencies
+      uses: actions/cache@v2
+      with:
+        path: ${{ steps.yarn-cache.outputs.dir }}
+        key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-yarn-
+    - name: Install dependencies
+      run: yarn install --no-ignore-optional --frozen-lockfile
+    - name: Audit dependencies
+      run: yarn audit || true
+      continue-on-error: true
+    - name: Build preview
+      run: yarn build:hugo --environment development --baseURL / --buildDrafts --buildFuture
+    - name: Lint
+      run: yarn lint
+    - name: Percy snapshots
+      uses: percy/snapshot-action@v0.1.2
+      env:
+        PERCY_TOKEN: ${{ secrets.PERCY_TOKEN }}