Local iframe source is not copied during building

[tuncbkose]

Description

I'd like to include a local HTML file in an iframe, but using something like

```{iframe} test.html
```

does not copy test.html to _build/html (with command myst build --html --execute).
On a related note, I couldn't get writing

<iframe src=test.html></iframe>

to work either.

Proposed solution

Add a check to iframe directive to see if the source is a local file.

Additional notes

myst v1.3.18