std.zig.fmt escaped string formatting recognizes single quote style
std.zig.fmt escaped string formatting recognizes single quote style

This introduces {'} to indicate escape for a single-quoted string,
and {} to indicate escape for a double quoted string.

Without this, there would be unnecessary \' inside double quoted
strings, and unnecessary \" inside single quoted strings.

Motivated by the llvm12 branch, in the new tool I am writing for
updating target CPU features.

zig-lang/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig,zig-lang/zig