Add tests, CI pipeline

[vieitesss]

Motivation

It is needed to add tests to the plugin in order to have some sort of a CI pipeline.

Solution

This would imply:

• Installing plenary.nvim to develop the tests.
• Build the CI pipeline:
  • Dagger (may be too much for know)
  • GitHub Actions
• Handling releases:
  • Release Please (may be too much as well)
  • Manual releases (don't like this)
  • In between method (?)