fix: convert enums to types

[achingbrain]

In #40 enums had const added to them. const enums are removed at compile time and replaced with numbers, e.g.:

const enum MyEnum {
    AValue,
    AnotherValue
}

function fn (arg: MyEnum) {
    console.info(arg)
}

fn(MyEnum.AnotherValue)

...compiles to:

function fn(arg) {
  console.info(arg);
}
fn(1 /* AnotherValue */);

Here we require strings to be passed to libdatachannel, so in #259 we exported objects with values that correspond to the enums so consuming code can import something they can use in PeerConnection method invocations, e.g.:

import { DescriptionType, PeerConnection } from 'node-datachannel'

const pc = new PeerConnection()
pc.setLocalDescription(DescriptionType.Offer, {
  // ...
})

In #278 the codebase was converted to TypeScript and a non-const enum crept back in. Now all the enums live in types.ts and since #300 all exports from types.ts are re-exported from index.ts.

Rollup was introduced as part of #278, but if you check the esm and cjs output dirs, tests.ts is not being transpiled to tests.js (I think because they aren't added to the default export of lib/index.js so would not be in the cjs version - types/lib/types.d.ts is present though, so tsc is doing it's job) and the re-export of the exports from tests.ts is being removed so enums are broken again at runtime.

The fix here is to give up on enums since they are a constant source of pain for consumers and change them to be types:

import { PeerConnection } from 'node-datachannel'

const pc = new PeerConnection()
pc.setLocalDescription('offer', {
  // ...
})

[achingbrain]

Needs linting fix from #322

[murat-dogan]

I am OK with that.
Thanks.