add a demo and tests to the horizontal rule extension

docs/src/demos/Extensions/HorizontalRule/index.spec.js

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+context('/api/extensions/horizontal-rule', () => {
+  beforeEach(() => {
+    cy.visit('/api/extensions/horizontal-rule')
+
+    cy.get('.ProseMirror').window().then(window => {
+      const { editor } = window
+      editor.setContent('<p>Example Text</p>')
+    })
+  })
+
+  describe('horizontal-rule', () => {
+    it('the button should add a horizontal rule', () => {
+      cy.get('.ProseMirror hr').should('not.exist')
+      cy.get('.demo__preview button:first').click({ force: true })
+      cy.get('.ProseMirror hr').should('exist')
+    })
+
+    it('the default markdown shortcut should add a horizontal rule', () => {
+      cy.get('.ProseMirror').window().then(window => {
+        const { editor } = window
+        editor.clearContent()
+
+        cy.get('.ProseMirror hr').should('not.exist')
+        cy.get('.ProseMirror').type('---', {force: true})
+        cy.get('.ProseMirror hr').should('exist')
+      })
+    })
+
+    it('the alternative markdown shortcut should add a horizontal rule', () => {
+      cy.get('.ProseMirror').window().then(window => {
+        const { editor } = window
+        editor.clearContent()
+
+        cy.get('.ProseMirror hr').should('not.exist')
+        cy.get('.ProseMirror').type('___ ', {force: true})
+        cy.get('.ProseMirror hr').should('exist')
+      })
+    })
+  })
+})

docs/src/demos/Extensions/HorizontalRule/index.vue

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+<template>
+  <div v-if="editor">
+    <button @click="editor.focus().horizontalRule()">
+      horizontalRule
+    </button>
+
+    <editor-content :editor="editor" />
+  </div>
+</template>
+
+<script>
+import { Editor } from '@tiptap/core'
+import { EditorContent } from '@tiptap/vue'
+import Document from '@tiptap/extension-document'
+import Paragraph from '@tiptap/extension-paragraph'
+import Text from '@tiptap/extension-text'
+import HorizontalRule from '@tiptap/extension-horizontal-rule'
+
+export default {
+  components: {
+    EditorContent,
+  },
+
+  data() {
+    return {
+      editor: null,
+    }
+  },
+
+  mounted() {
+    this.editor = new Editor({
+      extensions: [
+        Document(),
+        Paragraph(),
+        Text(),
+        HorizontalRule(),
+      ],
+      content: `
+        <p>This is a paragraph.</p>
+        <hr>
+        <p>And this is another paragraph.</p>
+        <hr>
+        <p>But between those paragraphs are horizontal rules.</p>
+      `,
+    })
+
+    window.editor = this.editor
+  },
+
+  beforeDestroy() {
+    this.editor.destroy()
+  }
+}
+</script>