Run GH Actions with Node v18 for fetch support (#35)

Running the tests on Node v18 fixes issues where Jest complains about
missing globals like Request/Response/Headers that are always available in
Remix environments so it's ok to just run the test on 18 intead of 16.

.github/workflows/main.yml

@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
-          node-version: 16
+          node-version: 18
           cache: 'yarn'
       - run: yarn install --frozen-lockfile
       - run: yarn lint
@@ -27,7 +27,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
-          node-version: 16
+          node-version: 18
           cache: 'yarn'
       - run: yarn install --frozen-lockfile
       - run: yarn test
@@ -38,7 +38,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
-          node-version: 16
+          node-version: 18
           cache: 'yarn'
       - run: yarn install --frozen-lockfile
       - run: yarn typecheck