Only show snippet markers for the current snippet on edit

[oykuyilmaz]

Issue #, if available:
Snippet markers continue as cascading:

tabstop-cascading.mp4

Description of changes:

In every tab next, adds markers only to the current snippet's tabstops.

fixed-markers.mp4

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d9814b4) 86.51% compared to head (12edfec) 86.52%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5478   +/-   ##
=======================================
  Coverage   86.51%   86.52%           
=======================================
  Files         584      584           
  Lines       42387    42410   +23     
  Branches     7056     7060    +4     
=======================================
+ Hits        36673    36696   +23     
  Misses       5714     5714           

Flag	Coverage Δ
unittests	86.52% <100.00%> (+<0.01%)	⬆️

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