std.fmt c specifier should print numbers as unicode codepoints

[Vexu]

It makes no sense to have unicode character literals but no way to format them.

test "" {
    @import("std").debug.warn("{c}\n", .{'あ'});
    // error: Cannot print integer that is larger than 8 bits as a ascii
}

The c specifier should format integers as unicode codepoints and a new specifier should be added for formatting them as their literal bytes.

This is how the Go, Python, Odin and Java handle c.

[data-man]

In #3970 I proposed 'u'.

u: output integer as an UTF-8 sequence. Integer type must have 21 bits at max.

But I prefer U+FFFD for invalid codepoints.

https://en.wikipedia.org/wiki/UTF-8#Invalid_byte_sequences

The standard also recommends replacing each error with the replacement character "�" (U+FFFD).

[andrewrk]

Implemented by @LemonBoy in #6390, landed in 473cb1f.