Add component: Cascader (#21410)

* Rename old cascader

* Change name of old cascader

* Add basic cascader without search

* Add basic cascader without search

* Flatten options to make it searchable

* Add regex search and make backspace work

* Add barebone search without styles

* Add SearchResult list

* Add search navigation

* Rewrite of cascader and add some things to SelectBase

* Make SelectBase controlllable

* Cleanup

* Add initial value functionality

* Add onblur to hand caret direction

* New storyboom format for ButtonCascader

* Add knobs to story

* Add story and docs for UnitPicker

* Make UnitPicker use Cascader

* Fix backspace issue and empty value

* Fix backspace issue for real

* Remove unused code

* Fix focus issue

* Change children to items and remove ButtonCascaderProps

* Remove local CascaderOption

* Fix failed test

* Revert UnitPicker changes and change format for ButtonCascader

* Fix failing tests

public/app/plugins/datasource/prometheus/components/PromQueryField.test.tsx

@@ -9,7 +9,7 @@ describe('groupMetricsByPrefix()', () => {
     expect(groupMetricsByPrefix(['foo_metric'])).toMatchObject([
       {
         value: 'foo',
-        children: [
+        items: [
           {
             value: 'foo_metric',
           },
@@ -22,7 +22,7 @@ describe('groupMetricsByPrefix()', () => {
     expect(groupMetricsByPrefix(['foo_metric'], { foo_metric: [{ type: 'TYPE', help: 'my help' }] })).toMatchObject([
       {
         value: 'foo',
-        children: [
+        items: [
           {
             value: 'foo_metric',
             title: 'foo_metric\nTYPE\nmy help',
@@ -44,7 +44,7 @@ describe('groupMetricsByPrefix()', () => {
     expect(groupMetricsByPrefix([':foo_metric:'])).toMatchObject([
       {
         value: RECORDING_RULES_GROUP,
-        children: [
+        items: [
           {
             value: ':foo_metric:',
           },