Improve iframe behavior

[gyoshev]

This PR resolves two problems:

• iframe height is polled at all times, deteriorating performance of all widgets
• iframes do not have an ID that can be used for snapshots and tests

After merging, the height and IDs can be modified via snippet metadata. The default height of snippets is now 280px, and the iframe id is inferred by the previous heading in the document, prefixed with example-. For example, "Basic Usage" implies that the next snippet will have id="example-basic-usage"

If a document heading changes, and there is a snippet after it, it would be nice to keep the old id after the change:

Before:

# Basic Usage

```ts
code
```

After:

 # Completely different title

 {% meta id:example-basic-usage %}
 ```ts
 code
 ```
 {% endmeta %}

/cc @ipeshev @yordanov @dmihaylo

[tsvetomir]

One hack less 🎉