Typecheck-only rule

[alexeagle]

So far we told users they have to use tsc_test if they just want a rule that errors when .d.ts inputs have typing errors. That's because Bazel won't run actions whose outputs aren't needed, and there are no outputs of a ts_project(srcs=["foo.d.ts"]) where we just want the typechecker to run.

But Bazel 6 has validation actions, as observed here #85 (comment)
so then we could have a typecheck-only rule, maybe it would be ts_project with some different behavior.