Define a separate Go module for the testify migration tool

`gty-migrate-from-testify` brings its own dependencies which are not used in general for users of `gotest.tools/v3/assert`.

By defining a new Go module for `gty-migrate-from-testify`, the dependency problem will be contained. `golang.org/x/tools` will disapear from the root `go.mod`.

How to do it:
```console
cd assert/cmd/gty-migrate-from-testify
go mod init gotest.tools/assert/cmd/gty-migrate-from-testify
go mod tidy
git add go.mod go.sum
sed -i= 's!gotest.tools/v3/assert/cmd/gty-migrate-from-testify!gotest.tools/assert/cmd/gty-migrate-from-testify/v1!' doc.go
git add doc.go
cd ../..
go mod tidy # Cleanup dependencies which were used only by gty-migrate-from-testify
git add go.mod go.sum
sed -i= 's!gotest.tools/v3/assert/cmd/gty-migrate-from-testify!gotest.tools/assert/cmd/gty-migrate-from-testify/v1!' assert/assert.go
git add assert/assert.go
git commit -m "assert/cmd/gty-migrate-from-testify: define Go module"
git tag -a assert/cmd/gty-migrate-from-testify/v1.0.0
git push --tags
```