feat: support env in path mapping

[Tolsee]

Just like vscode's env substitution, support ${env:ENV_VAR} in pathMappings

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 65.44%. Comparing base (23d1ef3) to head (9f53670).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1068      +/-   ##
==========================================
+ Coverage   65.37%   65.44%   +0.06%     
==========================================
  Files          17       18       +1     
  Lines        2279     2283       +4     
  Branches      538      539       +1     
==========================================
+ Hits         1490     1494       +4     
  Misses        789      789              

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