Improvements for spawning processes

[straight-shoota]

The stdlib APIs for spawning processes has some deficiencies for common use cases (convenience) and portability challenges.
This has lead to a number of discussions about improvements, which I'll try to summarize here.

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[straight-shoota]

So here's what I'd like to do, in roughly this order:

1. Introduce Process.new and Process.run overloads without a command parameter. The command parameter is implicitly args[0]. We can consider renaming args to a different name. These overloads should also drop the shell parameter.
   • Add Process.new with args instead of command, args #16681
   • Add Process.run(args) without command #16739
2. Introduce Process.run? overload which returns nil if executing the command fails. It should follow the new parameter design (no command, no shell).
   • Add Process.run? #16738
3. Introduce Process.capture
   • Add Process.capture #16773
   • Add IO::PrefixSuffixBuffer for capturing truncated streams #16774
4. Introduce %W array literal with interpolation (RFC 0021: Percent String Array Literal with Interpolation rfcs#21. [ ] (eventually) Deprecate the shell parameter. Potentially even the entire overloads with command parameter. We need to figure out what to do about command literals.Some of these steps should be rather straightforward. Others need some discussion about details.

[straight-shoota]

I've compiled all planned changes into a draft for RFC 0025.