request: provide Spago directly from `purescript/spago`

[peterbecich]

Feature suggestion/request:

• Rename spago-unstable to spago-next. This will continue to correspond to the latest release on NPM: https://www.npmjs.com/package/spago/v/next (I see this is updated like so: https://github.com/thomashoneyman/purescript-overlay/pull/114/files)

• Create a new package spago-unstable which points to the latest commit on https://github.com/purescript/spago.

This will enable purescript-overlay users to easily get the latest pre-release version of Spago.

Thank you