fix: Fixed handling surrogare characters in insert, replace, delete mode in Vim

[andrewnester]

Issue #, if available: #4108

Description of changes:

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[codecov]

Codecov Report

Base: 85.96% // Head: 85.99% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (38f893a) compared to base (357fcf5).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4934      +/-   ##
==========================================
+ Coverage   85.96%   85.99%   +0.03%     
==========================================
  Files         542      542              
  Lines       41398    41451      +53     
  Branches     6552     6554       +2     
==========================================
+ Hits        35587    35645      +58     
+ Misses       5811     5806       -5     

Flag	Coverage Δ
unittests	85.99% <100.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
src/keyboard/vim.js	89.82% <100.00%> (+0.13%)	⬆️
src/keyboard/vim_test.js	98.60% <100.00%> (+0.01%)	⬆️
src/edit_session.js	85.76% <0.00%> (+0.08%)	⬆️

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