Refactor: Avoid async when interfacing with the R shell

[Ellpeck]

Also known as: add entirely synchronous RShellExecutor class

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (75a417d) 71.03% compared to head (bd5a388) 71.24%.
Report is 4 commits behind head on main.

Files	Patch %	Lines
src/r-bridge/shell-executor.ts	87.01%	6 Missing and 4 partials ⚠️
src/r-bridge/shell.ts	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #615      +/-   ##
==========================================
+ Coverage   71.03%   71.24%   +0.20%     
==========================================
  Files         213      215       +2     
  Lines        6774     6858      +84     
  Branches     1053     1061       +8     
==========================================
+ Hits         4812     4886      +74     
- Misses       1685     1691       +6     
- Partials      277      281       +4     

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