Add new `go.mod` file

[dkotik]

Go module path is moving to a new path. At this time, it is best to create a new go.mod file. This also removes the need to provide alternative name for import.

Related:

• https://github.com/starfederation/datastar/pull/869
• https://github.com/starfederation/datastar/pull/871

[dkotik]

this will only get fixed after a tagged version is released, so module can be installed from the new path:

site/smoketests/merge_options_test.go:7:2: no required module provides package github.com/starfederation/datastar/sdk/go/datastar; to add it:
	go get github.com/starfederation/datastar/sdk/go/datastar

[bencroker]

We have this in the new site, so I’ll close here. Thanks for the PR!