Document Security Release Process

Closes #187 (closed)

Security Release Process

• I started adjusting the Security Release process for developers to the specifics to the gitlab-vscode-extension.
• The release process itself (i.e. how are we going to publish the extension to the marketplace) is not finalized, so there is a TODO left in the "release step" of the security release process.

Security harness

• Taken from https://gitlab.com/gitlab-org/gitter/webapp/-/blob/develop/scripts/security-harness.js
• We've translated the ruby script from scripts/security-harness to JavaScript in the gitlab-org/gitter/webapp, so we didn't introduce a dependency on ruby for a single script. The same logic applies here.