feat: support custom display placeholder content

[hehehai]

Background

In the environment of poor network, loading cdn script and initializing preview will be blank for a long time.

Solution

Implement custom loading animation for the initial placeholder content of iframe

() =>
      h(Repl, {
       	...
        previewOptions: {
          placeholderHTML: ` <style>
      html, body {
        width: 100%;
        height: 100%;
      }
      body, html {
        display: flex;
        align-items: center;
        justify-content: center;
      }
      .loading {
        font-size: 20px;
      }
    </style>
    <div class="loading">Loading...</div>`
        }
      })

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
repl	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 22, 2023 2:01pm