refactor: clean up rtxrc editing

[jdx]

This uses a OnceCell to help with creating the editable document and memoizing it. It also uses a Mutex which reduces the need to making things mutable. I was using get_edit() and get_or_init_edit() as a memoizeable and non-memoizable methods, this makes it so get_edit() is memoized and doesn't require a mutable reference to Rtxrc

cc @roele

[codecov]

Codecov Report

Base: 82.00% // Head: 81.95% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (56361a9) compared to base (fcb74d7).
Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
- Coverage   82.00%   81.95%   -0.06%     
==========================================
  Files          89       89              
  Lines        2845     2848       +3     
==========================================
+ Hits         2333     2334       +1     
- Misses        512      514       +2     

Impacted Files	Coverage Δ
src/config/config_file/rtxrc.rs	77.83% <81.48%> (-0.78%)	⬇️
src/cli/alias/set.rs	100.00% <100.00%> (ø)
src/cli/alias/unset.rs	100.00% <100.00%> (ø)
src/cli/settings/set.rs	81.81% <100.00%> (ø)
src/cli/settings/unset.rs	100.00% <100.00%> (ø)
src/cli/plugins/install.rs	97.67% <0.00%> (+0.05%)	⬆️

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