title: Cypress custom command is not recognized when invoked
body: <p>I've created the following custom command in my <code>cypress/support/commands.js</code> file.</p>
  <pre><code>Cypress.Commands.add(&quot;login&quot;, (username, password) =&gt; {
      cy.request({
          method: 'POST',
          form: true,
          url: '/test/login/',
          body: {'username': username, 'password': password}
      })
  })
  </code></pre>
  <p>I had tests passing and login working before moving the login functionality to this custom command. I'm invoking it in my spec with <code>cy.login(testuser, testpwd)</code>, but I'm getting the following error message: <code>TypeError: cy.login is not a function</code>. The <a href="https://docs.cypress.io/api/cypress-api/custom-commands.html#Syntax" rel="noreferrer">docs say</a> that /cypress/support/commands.js is loaded before any test files are evaluated, so I assumed that simply placing a custom command in there would make the command available.  I'm running the tests through the local (GUI) test runner.</p>