chore: substrings of length one

[ST-DDT]

Replaces the usages of string.substring of length 1 with string[index].

This makes it easier to detect when we switch to string.at() in v9 via unicorn rule. No longer necessary.

See also:

• prefer-at should also mark substrings of length 1 sindresorhus/eslint-plugin-unicorn#2203

[codecov]

Codecov Report

Merging #2496 (59cfa3e) into next (2ba232e) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #2496      +/-   ##
==========================================
- Coverage   99.58%   99.58%   -0.01%     
==========================================
  Files        2823     2823              
  Lines      255520   255520              
  Branches     1102     1101       -1     
==========================================
- Hits       254466   254463       -3     
- Misses       1026     1029       +3     
  Partials       28       28              

Files	Coverage Δ
src/modules/helpers/luhn-check.ts	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes