Add command which displays API breakages

[sylr]

Feature request

Checklist

• The feature / improvement you are suggesting overlaps with the purpose of gomod:
  facilitating the management of Go modules and their dependencies.
• You have examined various alternatives to the new feature / improvement you are suggesting
  and are describing the results in the description below.
• You have filled in the three sections below and deleted their corresponding placeholders
  texts.

Description

I suggested in a golang/go issue that go mod could use pkg.go.dev as source to assert the magnitude of breakages a series a commit in a project would have golang/go#37135

I believe that part of this could be implemented in https://github.com/Helcaraxan/gomod

Implementation suggestions

A what(?) command which would take a git commit range as input and would display all signatures changes:

$ gomod what v0.0.2..HEAD
Changes from v0.0.2 to HEAD modified following exported signatures:
  github.com/org/go-lib/pkg/pkg1.New(...) (modified)
  github.com/org/go-lib/pkg/pkg2.NewWithContext(...) (deleted)
  github.com/org/go-lib/pkg/pkg2.NewWithBackgroundContext(...) (added)
  github.com/org/go-lib/pkg/pkg2.Struct{...} (modified)

$ gomod what v0.0.2..HEAD --exit-code
# exit 0 if no breaking changes
# exit 1 if changes imply a new Minor version (only added signatures)
# exit 2 if changes imply a new Major version (any changes or deleted signatures)