Implement sf::String in terms of std::u32string

[ChrisThrasher]

Description

Closes #2470

This will make it easier to do things like use UTF-32 string literals since sf::String will have a constructor that perfectly matches that type.

[codecov]

Codecov Report

Merging #2480 (e5fcb91) into master (7004db1) will decrease coverage by 1.81%.
The diff coverage is 64.28%.

❗ Current head e5fcb91 differs from pull request most recent head 66c70af. Consider uploading reports for the commit 66c70af to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2480      +/-   ##
==========================================
- Coverage   25.84%   24.03%   -1.81%     
==========================================
  Files         226      226              
  Lines       19419    19405      -14     
  Branches     4714     4714              
==========================================
- Hits         5018     4664     -354     
- Misses      13868    14259     +391     
+ Partials      533      482      -51     

Impacted Files	Coverage Δ
include/SFML/System/String.hpp	100.00% <ø> (ø)
src/SFML/Network/Packet.cpp	32.93% <0.00%> (ø)
src/SFML/Window/OSX/HIDInputManager.mm	1.08% <0.00%> (ø)
src/SFML/Window/Unix/KeySymToUnicodeMapping.cpp	0.00% <0.00%> (ø)
src/SFML/Window/Unix/KeyboardImpl.cpp	0.00% <0.00%> (ø)
src/SFML/System/String.cpp	100.00% <100.00%> (+0.50%)	⬆️

... and 14 files with indirect coverage changes

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[Bromeon]

Thanks! Mostly good, 2 minor comments 🙂

[vittorioromeo]

Generally looks good to me, I'm in favour of this PR.