std.fmt.format breaks with UTF-8 `─` character as fill

[pinpox]

Zig Version

0.11.0

Steps to Reproduce and Observed Behavior

When using some UTF-8 characters to pad/fill the output, print() fails. In the example below, while padding with + signs works as expected, padding with ─ does not. The character ─ is a box drawing character, useful e.g. when drawing tables with text in the terminal.

const std = @import("std");

pub fn main() !void {
    // std.debug.print("{?s:+^20}\n", .{"test"});  // WORKS
    // Output: ++++++++test++++++++ 
    std.debug.print("{?s:─^20}\n", .{"test"}); // DOES NOT WORK
}

❯ zig run main.zig
/nix/store/2rnf5zzshg65zw7g82bph1i1dph2z2r6-zig-0.11.0/lib/zig/std/fmt.zig:268:17: error: expected . or }, found '�
                @compileError("expected . or }, found '" ++ [1]u8{ch} ++ "'");
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/nix/store/2rnf5zzshg65zw7g82bph1i1dph2z2r6-zig-0.11.0/lib/zig/std/fmt.zig:151:55: note: called from here
        const placeholder = comptime Placeholder.parse(fmt[fmt_begin..fmt_end].*);
                                     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

Expected Behavior

For the padding to work with any printable character.

[vinnichase]

Hey I also encountered this problem when printing tables. I'm a complete newbie to system programming and zig. I took it as a chance to learn more about the inner workings of zig and produced a fix. I'm not expecting an early merge but I'm happy to discuss my implementation and learn about the paradigms of zig.