[WIP] Break down `JxlDecoder` to better support streaming, preview, animation, etc.

[inflation]

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 27.91878% with 142 lines in your changes missing coverage. Please review.

Project coverage is 84.99%. Comparing base (421606b) to head (03de9a8).

Files with missing lines	Patch %	Lines
jpegxl-rs/src/decode/session.rs	23.07%	130 Missing :warning:
jpegxl-rs/src/decode.rs	0.00%	6 Missing :warning:
jpegxl-rs/src/decode/event.rs	72.72%	6 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #79       +/-   ##
===========================================
- Coverage   98.23%   84.99%   -13.25%     
===========================================
  Files          15       17        +2     
  Lines         849     1046      +197     
===========================================
+ Hits          834      889       +55     
- Misses         15      157      +142     

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