DEV: Move calendar date + time picker from local dates into core component (#23023)

This commit moves the calendar date and time picker shown in
the local dates modal into a core component that can be reused
in other places. Also add system specs to make sure there isn't
any breakages with this feature, and a section to the styleguide.

plugins/discourse-local-dates/test/javascripts/acceptance/local-dates-composer-test.js

@@ -130,7 +130,7 @@ acceptance("Local Dates - composer", function (needs) {
     await click(".delete-to-date");
 
     assert.notOk(
-      query(".pika-table .is-selected"),
+      query(".date-time-control.to.is-selected"),
       "deleting selected TO date works"
     );