build(deps-dev): update @vuepress/theme-default requirement

Updates the requirements on [@vuepress/theme-default](https://github.com/vuejs/vuepress/tree/HEAD/packages/@vuepress/theme-default) to permit the latest version.
- [Release notes](https://github.com/vuejs/vuepress/releases)
- [Changelog](https://github.com/vuejs/vuepress/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vuepress/commits/v1.4.0/packages/@vuepress/theme-default)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

package.json

@@ -14,7 +14,7 @@
     "@types/jest": "^25.1.4",
     "@types/node": "^13.1.0",
     "@vuepress/plugin-google-analytics": "^1.4.0",
-    "@vuepress/theme-default": "^1.3.0",
+    "@vuepress/theme-default": "^1.4.0",
     "@wessberg/rollup-plugin-ts": "^1.2.23",
     "conventional-changelog-cli": "^2.0.31",
     "husky": "^4.2.3",