Add index.d.cts file to improve CommonJS support

[ptesavol]

Currently using node-datachannel in a CommonJS typescript projects requires using very specific tsconfig settings in the importing project, namely:

 "compilerOptions": {
         "module": "CommonJS",
        "moduleResolution": "Node10"
    },

To make the import work with other "module" and "moduleResolution" settings, a index.d.cts file would be required in "lib/" This file can be a copy of the current index.d.ts file, and maybe it could be created in the "prepublishOnly" script.

The reason why a separate index.d.cts file is required seems to be related to this open issue in typescript: microsoft/TypeScript#52363

[murat-dogan]

Could you please confirm this PR solves the issue?
#244

[ptesavol]

Thank you for the swift reply, the PR solves the issue.

[ptesavol]

Hi, could you publish v 0.8.0 to NPM so we can take it into use?

[murat-dogan]

Version released