Variables: migrates data source variable type to React/Redux (#22770)

* Refactor: moves all the newVariables part to features/variables directory

* Feature: adds datasource type

* Tests: adds reducer tests

* Tests: covers data source actions with tests

* Chore: reduces strict null errors

public/app/features/variables/state/reducers.ts

@@ -0,0 +1,19 @@
+import { combineReducers } from '@reduxjs/toolkit';
+import { optionsPickerReducer, OptionsPickerState } from '../pickers/OptionsPicker/reducer';
+import { variableEditorReducer, VariableEditorState } from '../editor/reducer';
+import { variablesReducer } from './variablesReducer';
+import { VariableModel } from '../../templating/variable';
+
+export interface TemplatingState {
+  variables: Record<string, VariableModel>;
+  optionsPicker: OptionsPickerState;
+  editor: VariableEditorState;
+}
+
+export default {
+  templating: combineReducers({
+    optionsPicker: optionsPickerReducer,
+    editor: variableEditorReducer,
+    variables: variablesReducer,
+  }),
+};