public shell_quote, shell_unquote API

[StefanKarpinski]

There's a reason that Base.shell_escape, Base.shell_parse and Base.shell_split are not exported: they are kind of a wonky, not-quite right API, with bad names. I've highlighted some of the issues in #31989 (comment), including:

• Base.shell_escape does quoting not escaping
• Base.shell_parse should be pared with Base.shell_quote but it produces a very internal data structure that is not really useful outside of the implementation of backticks
• Base.shell_split is actually the rough inverse of Base.shell_escape but not quite: Base.shell_escape is String --> String while Base.shell_split is String --> Vector{String}
• There are also the variations: Base.shell_escape_posixly and hypothetically Base.shell_escape_dosly and Base.shell_escape_powerfully (for PowerShell, presumably, @vtjnash?)

It would be good to have an official shell quoting and unquoting API since as it is people are just using Base.shell_escape even though it's internal and unofficial. Maybe it belongs in a packages (or stdlib) but the current situation is a bit of a mess (my mess).

[JamesWrigley]

I seem to have inadvertently done this anyway in #53510 😅 I guess this can be closed now.