Hi! Thank you for developing this great library.

Is your feature request related to a problem? Please describe.
With modern javascript, it is common practice to use importmaps in the head of the document. This is not currently supported when creating dash app and external_scripts is the only relevant option.

<script type="importmap">
{
  "imports": { 'libname': url}
}
</script>

Describe the solution you'd like
Add an option to Dash application to add an importmap to page head.

Describe alternatives you've considered
I am currently using regular expressions to edit app.index_string, but this can be brittle and may no longer work with newer dash versions.

Additional context
I have tried the forums search, but there seem to be some issues searching for a specific string like "importmap". I am not 100% confident this has not been addressed there.