Add Process.capture

[straight-shoota]

Implement Process.capture, Process.capture? and Process.capture_result from RFC 0025.

This implements the basic capture behaviour, which may be enhanced in the future.
In particular, this patch misses truncation of the captured error stream which will be a follow-up (#16774).

Resolves #7171

[straight-shoota]

This interpreter failure is pretty bizarre:

In spec\std\process_spec.cr:835:5

 835 | it "doesn't capture closed stderr" do
       ^
Error: compiling -> do

  result = Process.capture_result(to_ary(shell_command("1>&2 echo hello")), :close)
  result.status.success?.should(be_true)
  result.output?.should(eq(""))
  result.error?.should(be_nil)
end


In spec\std\process_spec.cr:836:88

 836 | result = Process.capture_result(to_ary(shell_command("1>&2 echo hello")), error: :close)
                                                                                        ^
Error: BUG: missing downcast_distinct from Symbol to Process::Redirect (Crystal::SymbolType to Crystal::EnumType)

1. The example right before it uses exactly the same syntax, just with the output parameter. Both output and error have the same type restriction, though (Stdio). The other one should be analysed first, so why does it not fail?
2. It only fails on Windows, but not on Linux. The interpreter should behave exactly the same on both targets.Ah, but we're skipping process specs in the interpreter on anything but Windows because SIGCHLD doesn't work

[ysbaddaden]

I'm having second thoughts on #capture_result. Looking at the implementation, I feel like it's mostly duplicating #run with implicit output = IO::Memory.new and error = IO::Memory.new and that's it 🤔

[straight-shoota]

Yeah, these methods are all essentially convenience wrappers. Process.run(...) itself is just a shortcut for Process.new(...).wait.
But the important thing is that they extract that meaning for a commonly used behaviour and give it a name. Maybe the name capture_result is not ideal. But the method certainly does more than I would want to repeat on every call site.