Add coding guidelines and lint-fix script

- Add coding guidelines section to copilot instructions
- Specify rule about avoiding comments in unit tests
- Add lint-fix npm script for auto-fixing ESLint issues

.github/copilot-instructions.md

@@ -7,6 +7,10 @@ Please follow these guidelines when contributing:
 - Install dependencies: `npm install && npm run setup`
 - Build and bundle demo: `npm run build && npm run esbuild`
 
+## Coding guidelines
+
+- Do not write comments in unit tests unless the test is particularly complex. When they are complex, add the descriptions into the assertion calls where possible.
+
 ## Unit tests
 
 Unit tests are run with `yarn test-unit`:

package.json

@@ -42,6 +42,7 @@
     "test": "npm run test-unit",
     "posttest": "npm run lint",
     "lint": "eslint -c .eslintrc.json --max-warnings 0 --ext .ts src/ addons/",
+    "lint-fix": "eslint -c .eslintrc.json --fix --ext .ts src/ addons/",
     "lint-api": "eslint --no-eslintrc -c .eslintrc.json.typings --max-warnings 0 --no-ignore --ext .d.ts typings/",
     "test-unit": "node ./bin/test_unit.js",
     "test-unit-coverage": "node ./bin/test_unit.js --coverage",