Remove duplicate alphabets and replace seqan2::String

[eaasna]

1. Incorporate stellar3 headers instead of submodule
2. Replace seqan2::Dna and seqan3::dna4 with seqan2::alphabet_adaptor
3. Replace seqan2::String with std::span

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.44%. Comparing base (519f212) to head (3113d9b).

❗ Current head 3113d9b differs from pull request most recent head 1115648

Please upload reports for the commit 1115648 to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #109   +/-   ##
=======================================
  Coverage   94.44%   94.44%           
=======================================
  Files           2        2           
  Lines          72       72           
=======================================
  Hits           68       68           
  Misses          4        4           

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