Plugin system

[wolflu05]

As discussed in #98 it would be really nice to add plugin support to djlsp.

For defining plugins it the importlib metadata entrypoints: https://docs.python.org/3/library/importlib.metadata.html#entry-points can be used so that there can be not only first party plugins, but also plugins that can be loaded from sources outside of this repository.
Plugins should be able to inference in multiple ways with the LSP.

1. Data collection

Plugins should be able to:

• modify existing collected data
• add new collect data (e.g. discover custom templates, ...)
• collect custom data for plugin usage later (add custom data to the collected data that they can later use in the LSP)

The wagtail collector can the be moved into a contrib package.

2. Custom parser

Plugins should be able to add custom completion and hover rules. This would be useful to extract internal contrib things as well as for external projects using djlsp. These plugins then should have access to the data they have collected. E.g.

• Compatibility with django-components? #86
• [WIP] Add css class name completions #90
• inventree template editor (Custom collectors #98 (comment))

3. Provide live context

Plugins should be able to provide the actual context data for using in hover/completions to help the user decide on a specific variable. This can be used with jedi.Interpreter.