general updates to the setup action

[devoncarew]

We could likely use a general update to the action. Some things we'd want to include:

• use patch versions in our versioning scheme (even if they're always zero). currently we use major.minor, which is non-standard (#71)
• refactor the implementation of the action into Dart
• auto-detect macos arm; #59
• support version segment placeholder; #56
• consider revising and improving our caching for provisioning the dart sdk #2
• consider adding auto-caching for pub packages / the pub cache

Additionally, the implementation of this action is in bash; we may consider refactoring it into Dart; this will give us access to some std libraries from github - like for caching - and generally make the action easier to maintain.