plugin-uninstall: added --purge flag

[jdx]

this changes the behavior of uninstalls to now no longer remove installed plugins, see: https://github.com/jdx/rtx/issues/942

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (2796c45) 87.43% compared to head (e521f1e) 87.41%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #975      +/-   ##
==========================================
- Coverage   87.43%   87.41%   -0.03%     
==========================================
  Files         132      132              
  Lines       11591    11613      +22     
==========================================
+ Hits        10135    10151      +16     
- Misses       1456     1462       +6     

Files	Coverage Δ
src/cli/plugins/install.rs	92.04% <100.00%> (+0.09%)	⬆️
src/cli/plugins/ls.rs	93.75% <100.00%> (ø)
src/config/mod.rs	93.29% <100.00%> (ø)
src/plugins/external_plugin.rs	85.92% <100.00%> (-0.24%)	⬇️
src/plugins/mod.rs	75.22% <100.00%> (ø)
src/runtime_symlinks.rs	100.00% <100.00%> (ø)
src/toolset/tool_version_list.rs	96.22% <100.00%> (ø)
src/cli/plugins/uninstall.rs	96.15% <75.00%> (-3.85%)	⬇️
src/tool.rs	92.02% <79.16%> (-1.05%)	⬇️

... and 1 file with indirect coverage changes

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