feat: add check_username function

[Lucino772]

• Add url for check_username function
• Add check_username function
• Add tests for check_username function
• Update README

[codecov-commenter]

Codecov Report

Merging #30 (7b8df1b) into main (fb51a8a) will increase coverage by 0.53%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   48.10%   48.64%   +0.53%     
==========================================
  Files          29       29              
  Lines         952      958       +6     
  Branches      121      122       +1     
==========================================
+ Hits          458      466       +8     
+ Misses        480      479       -1     
+ Partials       14       13       -1     

Impacted Files	Coverage Δ
mojang/api/session.py	92.10% <100.00%> (+0.55%)	⬆️
mojang/api/urls.py	100.00% <100.00%> (ø)
mojang/api/helpers.py	100.00% <0.00%> (+6.89%)	⬆️

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