add an interactive demo and tests to the underline extension

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

@@ -0,0 +1,33 @@
+context('/api/extensions/underline', () => {
+  beforeEach(() => {
+    cy.visit('/api/extensions/underline')
+
+    cy.get('.ProseMirror').window().then(window => {
+      const { editor } = window
+      editor.setContent('<p>Example Text</p>')
+      editor.focus().selectAll()
+    })
+  })
+
+  describe('bold', () => {
+    it('the button should underline the selected text', () => {
+      cy.get('.demo__preview button:first').click({ force: true })
+      cy.get('.ProseMirror').contains('u', 'Example Text')
+    })
+
+    it('the button should toggle the selected text underline', () => {
+      cy.get('.demo__preview button:first').dblclick({ force: true })
+      cy.get('.ProseMirror u').should('not.exist')
+    })
+
+    it('the keyboard shortcut should underline the selected text', () => {
+      cy.get('.ProseMirror').type('{meta}u', {force: true})
+      cy.get('.ProseMirror').contains('u', 'Example Text')
+    })
+
+    it('the keyboard shortcut should toggle the selected text underline', () => {
+      cy.get('.ProseMirror').type('{meta}u', {force: true}).type('{meta}u', {force: true})
+      cy.get('.ProseMirror u').should('not.exist')
+    })
+  })
+})