vector tiles: style parallel edges

[flaktack]

Summary

The vector tile shows the edges, but the forward/backward edges overlap. A line-offset of half the line width is added to the style, so the directions are separated:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.86%. Comparing base (07c72ea) to head (b7563d6).
Report is 5 commits behind head on dev-2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #6134   +/-   ##
==========================================
  Coverage      69.85%   69.86%           
+ Complexity     17497    17494    -3     
==========================================
  Files           1976     1976           
  Lines          74710    74711    +1     
  Branches        7645     7643    -2     
==========================================
+ Hits           52191    52194    +3     
+ Misses         19868    19867    -1     
+ Partials        2651     2650    -1     

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[vesameskanen]

Amazingly simple!