Allow overrides for flash message styles

[sevenseacat]

When using views that set flash messages, eg. resettable - the styling of the flash messages should be customizable to fit with the rest of the app.

At the moment these are hardcoded here -

ash_authentication_phoenix/lib/ash_authentication_phoenix/templates/live.html.heex

Lines 3 to 16 in 112b86d

	class="fixed top-2 right-2 mr-2 w-80 sm:w-96 z-50 rounded-lg p-3 text-sm
	bg-emerald-100 dark:bg-emerald-200 text-emerald-800"
	role="alert"
	phx-click="lv:clear-flash"
	phx-value-key="info"
	>
	<%= Phoenix.Flash.get(@flash, :info) %>
	</p>
	<% end %>
	<%= if Phoenix.Flash.get(@flash, :error) do %>
	<p
	class="fixed top-2 right-2 mr-2 w-80 sm:w-96 z-50 rounded-lg p-3 text-sm
	bg-rose-100 dark:bg-rose-200 text-rose-900"