feat: support custom updater as object as well as path

[eemeli]

This adds support for defining the updater directly in a JS config file, rather than requiring it to be passed in as a path to be required.

Fixes #631

[eemeli]

It looks like the force-push didn't trigger the GH action? That should have fixed the linter complaints.

[eemeli]

Rebased on latest master, with test following new mock style.

[brettdh]

I also ran into this issue today, in the context of expo-community/standard-version-expo#17.

In the README.md in that diff, I wrote require.resolve where I meant require, but the idea was to import a function from the module and call it, returning a { readVersion, writeVersion } object, as a way of passing extra parameters to the updater.

[brettdh]

Glad to see this merged. It's been a couple weeks since #662 was opened; when do you think we'll see 9.1.0 released? I'd like to start building on this functionality in expo-community/standard-version-expo#17.