test(utils): Improve String.capitalize() and add tests

[marfavi]

Now emoji-aware & empty-string aware!

Closes #470

[codecov[bot]]

Codecov Report

Merging #472 (0ccc6a1) into develop (355e296) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #472   +/-   ##
========================================
  Coverage    67.20%   67.20%           
========================================
  Files          101      101           
  Lines         1122     1122           
========================================
  Hits           754      754           
  Misses         368      368           

Flag	Coverage Δ
unittests	67.20% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/core/extensions/string_extensions.dart	100.00% <100.00%> (ø)

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