[MISC] Rename the index char_type to alphabet_type.

[smehringer]

Resolves #1432

[codecov]

Codecov Report

Merging #1433 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1433   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files         232      232           
  Lines        8831     8831           
=======================================
  Hits         8619     8619           
  Misses        212      212

Impacted Files	Coverage Δ
include/seqan3/search/fm_index/fm_index.hpp	100% <ø> (ø)	⬆️
include/seqan3/search/fm_index/bi_fm_index.hpp	100% <ø> (ø)	⬆️
...lude/seqan3/search/fm_index/bi_fm_index_cursor.hpp	100% <100%> (ø)	⬆️
include/seqan3/search/fm_index/fm_index_cursor.hpp	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 36e09b3...0371c79. Read the comment docs.

[smehringer]

sorry about the force push. I forgot to push that one before requesting a review

[rrahn]

lgtm