child_process: options.shell validation

[Renegade334]

Based on an original PR at #54623, with modifications based on #56761 (comment).

child_process: validate exec's options.shell as string

While execFile and spawn perform validation on options.shell, exec just silently ignores invalid values without raising any error. This changes that behaviour to an explicit validation.

child_process: deprecate passing options.shell as empty string

Passing { shell: '' } to the process creation functions should never be intentional, and doing so has some funky behaviour:

• when passed to execFile/spawn: is equivalent to { shell: false }
• when passed to exec: breaks out of exec's shell-based behaviour and instead spawns the process directly, interpreting command as a file path

This adds a pending deprecation for this case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.23%. Comparing base (705bcc2) to head (b3322f3).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #58525   +/-   ##
=======================================
  Coverage   90.22%   90.23%           
=======================================
  Files         635      635           
  Lines      187653   187596   -57     
  Branches    36867    36858    -9     
=======================================
- Hits       169310   169268   -42     
- Misses      11101    11103    +2     
+ Partials     7242     7225   -17     

Files with missing lines	Coverage Δ
lib/child_process.js	97.78% <100.00%> (+0.03%)	⬆️

... and 33 files with indirect coverage changes

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