Add aria-modal attribute to react-modal types (#37980)

types/react-modal/index.d.ts

@@ -32,8 +32,12 @@ declare namespace ReactModal {
     }
 
     interface Aria {
+        /** Defines a string value that labels the current element. */
         labelledby?: string;
+        /** Identifies the element (or elements) that describes the object. */
         describedby?: string;
+        /** Indicates whether an element is modal when displayed. */
+        modal?: boolean | 'false' | 'true';
     }
 
     interface Props {

types/react-modal/react-modal-tests.tsx

@@ -50,7 +50,8 @@ class ExampleOfUsingReactModal extends React.Component {
     };
     const customAriaVariables = {
       labelledby: 'labelledby',
-      describedby: 'describedby'
+      describedby: 'describedby',
+      modal: true,
     };
     const customDataVariables = {
       dataOne: 'one',