[Feature request] Backport FormattableString/FormattableStringFactory

Missing in net 4.5 and earlier. Use case (taken from Meziantou’s blog post):

string EscapeCommandLineArgs(FormattableString formattableString)
{
    var args = formattableString.GetArguments()
                   .Select(arg => CommandLineBuilder.WindowsQuotedArgument(string.Format("{0}", arg)))
                   .ToArray();
    return string.Format(formattableString.Format, args);
}

...
var arg1 = "c:\\Program Files\\whoami.exe";
var arg2 = "Gérald Barré";
var commandLine = EscapeCommandLineArgs($"{arg1} {arg2}"); // "c:\Program Files\whoami.exe" "Gérald Barré"

[theraot]

Nuget Version 3.1.1

[ig-sinicyn]

Can confirm it works:)