feat: add prettier for formatting (#151)

.github/workflows/ci.yml

@@ -0,0 +1,25 @@
+name: Check conformance with Prettier
+
+on:
+  pull_request:
+
+jobs:
+  prettier:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Check out repo
+      uses: actions/checkout@v2
+
+    - name: Cache dependencies
+      uses: actions/cache@v2
+      with:
+        path: ~/.npm
+        key: npm-${{ hashFiles('package-lock.json') }}
+        restore-keys: npm-
+
+    - name: Install dependencies
+      run: npm ci --ignore-scripts
+
+    - name: Run prettier
+      run: |-
+        npm run lint:format

.pre-commit-config.yaml

@@ -0,0 +1,5 @@
+repos:
+- repo: https://github.com/pre-commit/mirrors-prettier
+  rev: v2.7.1
+  hooks:
+    - id: prettier

.prettierignore

@@ -0,0 +1,5 @@
+# Ignore everything:
+/*
+
+# Except src:
+!/src