Robustly handle package schema breaking changes

[candrewlee14]

When package schema changes without backwards compatibility, webman should be able to notify the user that webman is out of date, and then install the package from a commit of webman-pkgs that was compatible (instead of erroring out trying to install an invalid new schema).

This could be something like a compatibility.yml in the root of webman-pkgs:

webman_versions:
  - v0.8.0: [last compatible webman-pkgs commit SHA]
  - v0.9.0: [last compatible webman-pkgs commit SHA]

and an unspecified end would use the latest commit.