Create httpgd NPM package

[nx10]

The client base parts (API wrapper, polling/WebSocket connection handler, ...) should be in a separate NPM package to unify and simplify development.

[nx10]

GitHub repo: https://github.com/nx10/httpgd-js
NPM package: https://www.npmjs.com/package/httpgd

@ManuelHentschel if you have any feedback let me know, I will create a PR to vscode-R when this is ready.

[ManuelHentschel]

I really like this idea, and also the other improvements you recently pushed! I haven't had a chance yet to look into them in detail, but hopefully next week I can do so.

I'll be happy to work together on the vscode-R integration. If you have a plan, just let me know what needs to be done on the vscode side. Otherwise I'll take a look next week, and come up with some ideas myself :)

[nx10]

Working together would be great! There should not be much changes needed to get the current vscode plotviewer running with this. After that there is adding a vscode-native export dialog, and reworking your theming code with the new portable SVG renderer.

I will proceed with cleaning up and documenting the NPM package someday until next week so we can get started then.

[nx10]

I have begun working on the vscode changes which are more involved than I anticipated, but help me a lot to generalize the httpgd-js implementation. I will create a WIP PR as soon as I have a working prototype.