Refactor for easier updating

[shyamd]

This is a major breaking refactor that accomplishes a few things:

• Encapsulates Citation and CitationBlock to make the process of extracting various types of citations from markdown easier to test and validate
• Encapsulates Registry into an abstract class with a clearer interface
• Implements a SimpleRegistry that uses Pybtex for basic citations conversion
• Implements a PandocRegistry that uses Pandoc and can handle CSL styles
• Automates detecting inline vs footnote styles in PandocRegistry
• Cleans up plugin tests into clear integration tests
• Implements an example that can be used to locally test features with full integration into mkdocs

As part of this there are some breaking changes:

• Removes cite_inline as this is automatically detected from the CSL file now
• Removes compatibility with old versions of pandoc<=2.11

[codecov]

Codecov Report

Attention: Patch coverage is 93.44262% with 16 lines in your changes missing coverage. Please review.

Project coverage is 92.11%. Comparing base (dd0695f) to head (efcb83a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mkdocs_bibtex/registry.py	90.97%	12 Missing ⚠️
src/mkdocs_bibtex/citation.py	95.83%	2 Missing ⚠️
src/mkdocs_bibtex/plugin.py	96.22%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #285      +/-   ##
==========================================
- Coverage   94.30%   92.11%   -2.20%     
==========================================
  Files           3        6       +3     
  Lines         246      317      +71     
==========================================
+ Hits          232      292      +60     
- Misses         14       25      +11     

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