[Module API] Filter which entrypoints plugins run in

Feature Request

Add a way to filter which entrypoints plugins are added to.

Specifically, modify the addWxtPlugin function like so:

export default defineWxtModule((wxt) => {
  addWxtPlugin(
    wxt,
    "wxt-module-analytics/client-plugin", 
    (entry /* or `group` */) => {
      return true
    },
  );
});

The third parameter should be optional and default to the current behavior of running the plugin in all entrypoints.

Is your feature request related to a bug?

If so, add a link here. If not, write "N/A"

Start with the addWxtPlugin implementation and its Module API reference, especially how plugin entrypoints are currently selected. Trace the existing all-entrypoints behavior, then verify that an omitted third parameter preserves it while a predicate filters the selected entrypoints as described.