build(deps-dev): bump @vuepress/plugin-back-to-top from 1.3.1 to 1.4.0

Bumps [@vuepress/plugin-back-to-top](https://github.com/vuejs/vuepress/tree/HEAD/packages/@vuepress/plugin-back-to-top) from 1.3.1 to 1.4.0.
- [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/plugin-back-to-top)

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

package.json

@@ -70,7 +70,7 @@
     "@types/jest": "^25.1.4",
     "@types/node": "^13.9.1",
     "@vue/composition-api": "^0.4.0",
-    "@vuepress/plugin-back-to-top": "^1.3.1",
+    "@vuepress/plugin-back-to-top": "^1.4.0",
     "@vuepress/plugin-pwa": "^1.3.1",
     "axios": "^0.19.2",
     "brotli": "^1.3.2",

yarn.lock

@@ -1739,10 +1739,10 @@
   dependencies:
     lodash.debounce "^4.0.8"
 
-"@vuepress/plugin-back-to-top@^1.3.1":
-  version "1.3.1"
-  resolved "https://registry.yarnpkg.com/@vuepress/plugin-back-to-top/-/plugin-back-to-top-1.3.1.tgz#51fbb043845b46a66e85f66ae099041c76f08abc"
-  integrity sha512-vjfYqSnlZy7Z1Hmre7rw3crtUk9nVUJw+fG0CKp5VXLdBnzrACZznxrFw1BhuzqnV9UT3vq6n8MTlcbpq2Xs8g==
+"@vuepress/plugin-back-to-top@^1.4.0":
+  version "1.4.0"
+  resolved "https://registry.yarnpkg.com/@vuepress/plugin-back-to-top/-/plugin-back-to-top-1.4.0.tgz#baca4daf8de469544956bb21903dbeba691d40e4"
+  integrity sha512-tFsFx/jIoCh3hKWIDYioQyqhF+OJ+3QYAyPSJsvIadmOiS1Op33MlPVurWxr+J5hGlsvnOEyQqT9++mGh3N2KA==
   dependencies:
     lodash.debounce "^4.0.8"