fix: export DescriptionType enum values

[achingbrain]

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.

[achingbrain]

That is, I have the following code:

import { DescriptionType } from 'node-datachannel'

peerConnection.setLocalDescription(DescriptionType.offer)

With the current release:

// index.d.ts
export const enum DescriptionType {
...

I get a TS compilation error:

Cannot access ambient const enums when 'verbatimModuleSyntax' is enabled.ts(2748)

With:

// index.d.ts
export enum DescriptionType {
...

// index.js
// no DescriptionType export

TS is happy but JS is not:

import { DescriptionType } from 'node-datachannel'
         ^^^^^^^^^^^^^^^
SyntaxError: The requested module 'node-datachannel' does not provide an export named 'DescriptionType'

With what libdatachannel actually wants:

peerConnection.setLocalDescription('offer')

I get:

Argument of type '"offer"' is not assignable to parameter of type 'DescriptionType | undefined'.ts(2345)

With the changes in this PR TS, JS and C++ are all happy.

[murat-dogan]

I am OK with that.
And also we need to add other enums then.