Add a CI workflow to publish new releases after a tag is pushed

[Kobzol]

Context: rust-lang/infra-team#117

This repo has write access by t-compiler and t-compiler-contributors, and currently this workflow creates a release after a push of a tag or manually through workflow_dispatch. If we don't want to give right to publish to t-compiler-contributors, then we can remove the workflow_dispatch trigger, and create a branch protection to e.g. v-* tags that would only allow t-compiler to push.

Requires a new secret CARGO_REGISTRY_TOKEN to be added to the secrets of this repository, which has scoped access to publish rustc-demangle.

[Kobzol]

CC @jdno, could you please also add the token here? Thanks!

[jdno]

CARGO_REGISTRY_TOKEN has been added. 👍