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)