feat: add support for global config

[calebdw]

This adds support for a global config under the user's XDG_CONFIG (typically ~/.config/phpantom_lsp). Project level configs take precedence

Thanks!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.01961% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.58%. Comparing base (3f99b6e) to head (bbebcaa).

Files with missing lines	Patch %	Lines
src/config.rs	99.01%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   87.56%   87.58%   +0.01%     
==========================================
  Files         121      121              
  Lines       60431    60512      +81     
==========================================
+ Hits        52919    53001      +82     
+ Misses       7512     7511       -1     

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