DEV: move select-kit plugin-api file to lib directory (#32507)

Lift and shift, this file ceased to be a mixin as of
https://github.com/discourse/discourse/pull/28467.

app/assets/javascripts/discourse/app/lib/plugin-api.gjs

@@ -157,7 +157,7 @@ import {
 } from "discourse/widgets/widget";
 import { addImageWrapperButton } from "discourse-markdown-it/features/image-controls";
 import { CUSTOM_USER_SEARCH_OPTIONS } from "select-kit/components/user-chooser";
-import { modifySelectKit } from "select-kit/mixins/plugin-api";
+import { modifySelectKit } from "select-kit/lib/plugin-api";
 
 const DEPRECATED_POST_STREAM_WIDGETS = [
   "actions-summary",

app/assets/javascripts/discourse/tests/integration/components/select-kit/api-test.gjs

@@ -8,7 +8,7 @@ import selectKit, {
 } from "discourse/tests/helpers/select-kit-helper";
 import ComboBox from "select-kit/components/combo-box";
 import SingleSelect from "select-kit/components/single-select";
-import { clearCallbacks } from "select-kit/mixins/plugin-api";
+import { clearCallbacks } from "select-kit/lib/plugin-api";
 
 module("Integration | Component | select-kit/api", function (hooks) {
   setupRenderingTest(hooks);

app/assets/javascripts/select-kit/addon/components/select-kit.js

@@ -21,7 +21,7 @@ import { i18n } from "discourse-i18n";
 import {
   applyContentPluginApiCallbacks,
   applyOnChangePluginApiCallbacks,
-} from "select-kit/mixins/plugin-api";
+} from "select-kit/lib/plugin-api";
 import UtilsMixin from "select-kit/mixins/utils";
 
 export const MAIN_COLLECTION = "MAIN_COLLECTION";