Add feature flags `jsNode` and `jsWeb`

[marzipankaiser]

Currently distinguished by the Backend creating the JavaScript generator object (backend). Untested.

[marzipankaiser]

@jiribenes I vaguely remember you needed to change some things in the js code for your presentation tool - What was that again? We could integrate those changes here for the JSWeb backend.

[marzipankaiser]

This is related to #405: When introducing a js-web backend, we should have js code that can run in the browser.

[b-studios]

Maybe this also needs to be changed, when merging this PR: http://effekt-lang.org/docs/getting-started#compiling-programs-with-effekt

[marzipankaiser]

Maybe this also needs to be changed, when merging this PR: http://effekt-lang.org/docs/getting-started#compiling-programs-with-effekt

This should remain unchanged - apart from some names in code, the js (node) backend stays the same. That is, unless we want to also say something about js-web.

[marzipankaiser]

Marking this as ready for review. - While interrelated to #511, the hotfix works for now™️ and this could be merged first. Then, we could also start adjusting the stdlib where appropriate.