docs: add GitHub Release badge after first release

[SebTardif]

After the first GitHub release is published, add a Release badge to the README:

[![Release](https://img.shields.io/github/v/release/patchloom/patchloom-vscode?logo=github&sort=semver)](https://github.com/patchloom/patchloom-vscode/releases/latest)

Per badge hygiene rule #3 (do not add badges that show embarrassing state), this should only be added after a release exists. Currently the badge would show "no releases" which hurts more than it helps.

Identified by repo-badges skill audit.

[SebTardif]

An Open VSX badge has also been added alongside the Marketplace badge in PR #73. The GitHub Release badge will become relevant once release-please creates the first release tag. Both will resolve after the first publish.

[SebTardif]

Closed by PR #80. GitHub Release badge added to README.