fix: export DescriptionType enum values

At compliation time the values of a `const enum` in TS get changed
to numbers which breaks JS APIs that expect the string values.

At runtime anything in a `.d.ts` file is not available, so the
solution is to export enum definitions from the `.d.ts` file as a
regular `enum` (no `const`) and export the matching values from the
implementation file as a plain object.

lib/index.d.ts

@@ -104,7 +104,7 @@ export interface SelectedCandidateInfo {
 }
 
 // Must be same as rtc enum class Direction
-export const enum Direction {
+export enum Direction {
     SendOnly = 'SendOnly',
     RecvOnly = 'RecvOnly',
     SendRecv = 'SendRecv',

lib/index.js

@@ -41,3 +41,11 @@ export default {
     ...nodeDataChannel,
     DataChannelStream,
 };
+
+export const DescriptionType = {
+    unspec: 'unspec',
+    offer: 'offer',
+    answer: 'answer',
+    pranswer: 'pranswer',
+    rollback: 'rollback',
+};