extra_body_script() support for script type="module"

[simonw]

Follows #1186. The extra_body_script() plugin hook should provide a mechanism for specifying that the script should use <script type="module">.

Relevant docs: https://docs.datasette.io/en/stable/plugin_hooks.html#extra-body-script-template-database-table-columns-view-name-request-datasette

[simonw]

This plugin hook currently returns a string of JavaScript. It could optionally return this instead of a string:

{
  "script": "string of JavaScript goes here",
  "module": true
}