chore(lint): add a minimal eslint configuration (stenciljs#3369)

this commit adds a very basic eslint setup. linting can be done through
the command line, an npm script, and through ci as a result of this
commit.

the team is working on adding a new CI check for unused functions
that which will depend on the configuration already being checked in
on `main`, so I wanted to introduce a separate PR just to bring this in.

the only rule turned on (for now) is the unused-vars rule.

STENCIL-15: add initial eslint configuration

Author: alicewriteswrongs

.eslintrc.js

@@ -0,0 +1,18 @@
+module.exports = {
+  root: true,
+  parser: '@typescript-eslint/parser',
+  plugins: ['@typescript-eslint'],
+  extends: [
+    // including prettier here ensures that we don't set any rules which will conflict
+    // with Prettier's formatting. Keep it last in the list so that nothing else messes
+    // with it!
+    'prettier',
+  ],
+  rules: {
+    '@typescript-eslint/no-unused-vars': ['error', {
+      "argsIgnorePattern": "^_",
+      // TODO(STENCIL-452): Investigate using eslint-plugin-react to remove the need for varsIgnorePattern
+      "varsIgnorePattern": "^(h|Fragment)$" 
+    }]
+  },
+};

.github/workflows/format.yml renamed to .github/workflows/lint-and-format.yml

@@ -1,4 +1,4 @@
-name: Format Stencil (Check)
+name: Lint and Format Stencil (Check)
 
 on:
   workflow_call:
@@ -19,3 +19,6 @@ jobs:
       - name: Prettier Check
         run: npm run prettier.dry-run
         shell: bash
+
+      - name: ESLint
+        run: npm run lint

.github/workflows/main.yml

@@ -18,9 +18,9 @@ jobs:
     name: Build
     uses: ./.github/workflows/build.yml
 
-  format:
-    name: Format
-    uses: ./.github/workflows/format.yml
+  lint_and_format:
+    name: Lint and Format
+    uses: ./.github/workflows/lint-and-format.yml
 
   analysis_tests:
     name: Analysis Tests