Add go.mod

[piggynl]

Without go.mod, go build and go test fails with error:

go: cannot find main module, but found .git/config in /work/nfnt-resize
        to create a module there, run:
        go mod init

Since 1.16, golang will build packages in module-aware mode by default, see: https://go.dev/blog/go116-module-changes#modules-on-by-default

[caojen]

It's so sad that nfnt/resize is no longer being updated. see: #63

[piggynl]

Thank you for the information 😺