feat: setting manifest with json script

current:
```html
<api-viewer src="path/to/manifest.json"></api-viewer>
```
desired (in addition to src, as an option):
```html
<api-viewer>
  <script slot="manifest" type="application/json">
  { "schemaVersion": 1.0.0, "readme": "..." }
  </script>
</api-viewer>
```

or:

```js
document.querySelector('api-viewer').manifest = { "schemaVersion": 1.0.0, "readme": "..." }
```