refactor: handling functions for cross-platform compatibility

[appleboy]

• Change the rmcmd and mkdircmd functions to accept an OS parameter
• Remove command_windows.go file
• Modify removeDestFile and Exec functions to use the OS parameter
• Add OS detection logic to removeAllDestFile and Exec functions
• Modify TestRemoveDestFile function to use the OS parameter

fix https://github.com/appleboy/drone-scp/pull/119

[codecov-commenter]

Codecov Report

Merging #166 (4506229) into master (8b578d1) will increase coverage by 0.64%. The diff coverage is 69.23%.

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@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   38.11%   38.76%   +0.64%     
==========================================
  Files           4        4              
  Lines         606      632      +26     
==========================================
+ Hits          231      245      +14     
- Misses        365      375      +10     
- Partials       10       12       +2     

Impacted Files	Coverage Δ
command.go	57.14% <57.14%> (-42.86%)	:arrow_down:
plugin.go	85.45% <76.00%> (-1.42%)	:arrow_down:

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