[dev.cmdgo] cmd/internal/str: add utilities for quoting and splitting args

JoinAndQuoteFields does the inverse of SplitQuotedFields: it joins a
list of arguments with spaces into one string, quoting arguments that
contain spaces or quotes.

QuotedStringListFlag uses SplitQuotedFields and JoinAndQuoteFields
together to define new flags that accept lists of arguments.

For golang/go#41400

Change-Id: I4986b753cb5e6fabb5b489bf26aedab889f853f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/334731


Trust: Jay Conrod <jayconrod@google.com>
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>