Set up npm run lint to call markdownlint.

This makes it easier for other developers to run markdownlint, and
checking in package-lock.json ensures that we're all using the same
version.

Futhermore, this commit calls `npm run lint` directly instead of using
the GitHub Action because the action doesn't seem to be working.

Author: MarkLodato

.github/workflows/lint.yml

@@ -1,12 +1,11 @@
 name: Lint
-on: [pull_request]
+on: [workflow_dispatch, pull_request]
 permissions: read-all
 jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-    - name: Markdown Lint
-      uses: nosborn/github-action-markdown-cli@6e34c67b8bd3406dd8ee6eeba353ca4c66793501 # v2.0.0
-      with:
-        files: .
-        config_file: ".markdownlint.yaml"
+    - uses: actions/checkout@v2
+    - uses: actions/setup-node@v2
+    - run: npm ci --ignore-scripts
+    - run: npm run lint --silent

.gitignore

@@ -1,2 +1,3 @@
 .sass-cache
 _site
+node_modules