feat: adds classes and methods for sequence dictionaries

[nh13]

Fixes #152

[codecov]

Codecov Report

Attention: Patch coverage is 93.92523% with 13 lines in your changes missing coverage. Please review.

Project coverage is 89.02%. Comparing base (2261825) to head (408fec4).
Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
fgpyo/fasta/sequence_dictionary.py	93.92%	8 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
+ Coverage   88.44%   89.02%   +0.58%     
==========================================
  Files          16       17       +1     
  Lines        1791     2005     +214     
  Branches      381      437      +56     
==========================================
+ Hits         1584     1785     +201     
- Misses        136      144       +8     
- Partials       71       76       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tfenne]

Mostly 👍 but with a few thoughts/questions.

[tfenne]

A few minor comment, but good to go.