feat(config): enhance config file and data dir retrieval

[qwqcode]

• Add xdg package for XDG base directory support
• Update RetrieveConfigFile to search in XDG_CONFIG_HOME and /etc
• Add RetrieveDataDir to find data directory in XDG_DATA_HOME and /var/lib
• Improve comments for better understanding of search order

The preparatory work for PR: #991

[codecov]

Codecov Report

Attention: Patch coverage is 51.28205% with 19 lines in your changes missing coverage. Please review.

Project coverage is 32.24%. Comparing base (34d7936) to head (391ebb6).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/base.go	0.00%	18 Missing ⚠️
internal/config/base.go	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #992      +/-   ##
==========================================
+ Coverage   32.09%   32.24%   +0.14%     
==========================================
  Files         206      206              
  Lines        6857     6888      +31     
==========================================
+ Hits         2201     2221      +20     
- Misses       4502     4513      +11     
  Partials      154      154              

Flag	Coverage Δ
go	32.24% <51.28%> (+0.14%)	⬆️

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