feat: introduce logic branching based on GitLab version and use it for snippets

This MR introduces a simple utility function that can switch between different implementations based on the GitLab version that the extension connects to.

This MR also applies this utility to two problems:

• checking version at the start to warn user that their version is not supported
• switch implementation of retrieving snippet content (left behind by Inserting snippets is broken for newly created snippets)

Related to #428 (closed)