Consider supporting redirecting stderr to stdout

[lread]

Problem

Babashka/process currently easily supports controlling destinations for stderr and stdout.

But... it does not currently easily support merging stderr and stdout to a single output stream.
This can be very useful when you don't care about stderr vs stdout but do care about capturing all output in the order it occurs over time.

JDK Support

ProcessBuilder includes redirectErrorStream.
Docs on its effect are found in the ProcessBuilder class JavaDoc.

Explore Current Behaviour

Using the current release 0.4.16 of babashka/process, and a wee bb outerr.clj script to explore:

(println "I am written to stdout")

(binding [*out* *err*]
  (println "I am written to stderr"))

Today we can easily get stderr and stdout separately:

(require '[babashka.process :as p])

(-> (p/shell {:out :string :err :string} "bb outerr.clj")
    (select-keys [:out :err]))
;; => {:out "I am written to stdout\n", :err "I am written to stderr\n"}

But merging stderr to stdout, while possible, is a bit lower level than it could be:

(let [pb (p/pb {:out :string} "bb outerr.clj")
      pb-obj (:pb pb)]
  (.redirectErrorStream pb-obj true)
  (:out @(p/start pb)))
;; => "I am written to stdout\nI am written to stderr\n"

Proposed API change

@borkdude and I hashed out syntax on Slack and he came up with the succinct :err :out.
Nice!

With this new syntax, the lower-level example above could be re-expressed like so:

(-> (p/shell {:out :string :err :out} "bb outerr.clj")
    :out)
;; => "I am written to stdout\nI am written to stderr\n"

The :err :out syntax will be applicable to all API fns where :err is accepted in opts.

Next Steps

After @borkdude eyeballs the above, I'll proceed with a PR for review that includes both code and doc updates.

[borkdude]

Eyeballed!