add basic highlight extension

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

@@ -0,0 +1,51 @@
+context('/api/extensions/highlight', () => {
+  before(() => {
+    cy.visit('/api/extensions/highlight')
+  })
+
+  beforeEach(() => {
+    cy.get('.ProseMirror').then(([{ editor }]) => {
+      editor.setContent('<p>Example Text</p>')
+      editor.selectAll()
+    })
+  })
+
+  it('the button should highlight the selected text', () => {
+    cy.get('.demo__preview button:first')
+      .click()
+
+    cy.get('.ProseMirror')
+      .find('mark')
+      .should('contain', 'Example Text')
+  })
+
+  it('the button should toggle the selected text highlighted', () => {
+    cy.get('.demo__preview button:first')
+      .click()
+
+    cy.get('.ProseMirror')
+      .type('{selectall}')
+
+    cy.get('.demo__preview button:first')
+      .click()
+
+    cy.get('.ProseMirror')
+      .find('mark')
+      .should('not.exist')
+  })
+
+  it('the keyboard shortcut should highlight the selected text', () => {
+    cy.get('.ProseMirror')
+      .trigger('keydown', { modKey: true, key: 'e' })
+      .find('mark')
+      .should('contain', 'Example Text')
+  })
+
+  it('the keyboard shortcut should toggle the selected text highlighted', () => {
+    cy.get('.ProseMirror')
+      .trigger('keydown', { modKey: true, key: 'e' })
+      .trigger('keydown', { modKey: true, key: 'e' })
+      .find('mark')
+      .should('not.exist')
+  })
+})