[wip] [5 of #107] add dynamic options

[bollwyvl]

This encapsulates the state that does change, as highlighted in #124, in a single, relatively lenient object, and changes most function signatures to accept inputs, options or both.

• continues [do not merge] major refactor of downloads, environment creation #107

@bollwyvl: I haven't tested this one out. 🤞

On the next PR

refactor the installer provider

One of the first "irregularities" is that runInstaller is allowed to calculate its own outputPath, which requires access to mutable state. If we split up that behavior, we can get a lot of reuse, where it will be more straightforward to add additional "well-known" installer providers.

The idea here is to break up the installer provisioning step even more. There should be a significant net negative line count in setup.ts, and encapsulate the logic more succinctly along with its effects.

This is where we want to leave enough breathing room to handle micromamba #75 without actually building it right now.

[goanpeca]

LGTM @bollwyvl gave it a quick look :)

[bollwyvl]

Yeah, had another look I don't see anything else glaring at this step.