Improve repository.

- add kokuwa checkstyle
- add dependabot
- add dependency documentation
- add code owners
- add codeql
- add license badge

.github/workflows/dependabot.yaml

@@ -0,0 +1,22 @@
+name: Dependabot
+
+"on": pull_request_target
+
+jobs:
+  auto-merge:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - uses: dependabot/fetch-metadata@v1.1.1
+        id: metadata
+        with:
+          github-token: "${{ secrets.GIT_ACTION_TOKEN }}"
+      - run: gh pr review --approve "$PR_URL"
+        if: ${{ steps.dependabot-metadata.outputs.update-type != 'version-update:semver-major' }}
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{ secrets.GIT_ACTION_TOKEN }}
+      - run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{ secrets.GIT_ACTION_TOKEN }}