Document how to use javascript modules for clientside functions

[jowlo]

The PR https://github.com/plotly/dash/pull/2730 will introduce the possibility to use javascript modules for clientside functions. To be loaded as module the file in the asset folder need to end with .mjs.

For those files, the attribute type=module is automatically added to the script tag.

Mentioning this in the clientside callbacks section of the docs would be helpful for users looking to import modules and use larger external projects that are commonly packaged as modules.