[TypeSpec Validation] Validate specs in parallel to improve perf

I believe `Validate-TypeSpec.ps1` could be improved to validate specs in parallel, either by default or as an option. Would significantly improve perf when validating many specs on a multi-core machine.

I believe all specs under the same top-level folder must be validated in sequence, to prevent possible conflicts with multiple specs modifying the same file(s), and because "git diff" is run at the top-level folder level.

But all the top-level folders should be able to be validated in parallel.