Make specifier 's' work for struncts to print contained 'strings' as strings.

[jonasdiemer]

This is an alternative to #21330 to enable "more intuitive" formatting of structs.

Again, please consider it as "request for comment". I find it rather intuitive to be able to format structs as string, with the effect of nested strings being formatted as string.

This is a prototype, works for something like:

const myStruct = struct { name: []const u8, age: u8 };
const s = myStruct{ .name = "Dude", .age = 42 };
std.debug.print("struct: {}\n", .{s});
std.debug.print("struct s: {s}\n", .{s});

which results in

struct: main.myStruct{ .name = { 68, 117, 100, 101 }, .age = 42 }
struct s: main.myStruct{ .name = "Dude", .age = 42 }

If this makes sense to add, I can spend time to make sure to cover all cases, refactor, and add/adjust tests (pointers welcome, this is my first look into zig std lib).