Add support for global REPL

[eahlberg]

Description of the change

Adds support for starting a REPL within a folder which has not been setup as a spago project. Fixes #168.

Checklist:

• Added the change to the "Unreleased" section of the changelog
• Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

[eahlberg]

@f-f I decided to take a look at this and I tried to implement it according to #168 (comment). However, I’m a bit unsure of a couple of things (in addition to the comment above):

• Any idea for how this could be tested? I couldn’t find a test for spago repl, is there a specific reason for that?
• I saw that PackageSet.ensureFrozen was called in Config.ensureConfig, which is not used for this, but I'm not sure what freezing a package set means. Should it be used here?

[f-f]

@eahlberg looks good so far! I added some minor comments, and to answer your other points:

• let's skip tests for this one as I don't have ideas about how to test spago repl
• "freezing" a Dhall file means to add sha256 hashes to the remote imports (the package set URL in our case). Since in this case we're initing a new project it should be a no-op as the template already has hashes, so it should be fine to use ensureConfig.
  We have a small section about freezing in the README, but it's very small as it's a detail that's supposed to be automated away from the user. However in there you can find a link to a longer description of Dhall's freezing mechanism

[f-f]

I just noticed that in addition to the install flags we'll also need to add the --package flag to add packages as specified in #168. It's going to look exactly as the packageNames argument:
https://github.com/spacchetti/spago/blob/92e9c64bdde0373a64514def96b7f78738601909/app/Spago.hs#L158
..except it will be an opt instead of an arg

[eahlberg]

@f-f thanks for the feedback! 🙇‍♂️ I think I’ve addressed all of it and I also left a couple of questions above, so feel free to take a look when time permits.

[f-f]

Just a minor suggestion but otherwise looks great! 👏

I added you as collaborator so you can merge when you're ready 🙂

[eahlberg]

@f-f cool, thanks!