format: do not force user to provide an alignment field when it's not necessary (#19049)

* format: fix default character when no alignment

When no alignment is specified, the character that should be used is the
fill character that is otherwise provided, not space.

This is closer to the default that C programmers (and other languages)
use: "04x" fills with zeroes (in zig as of today x:04 fills with spaces)

Test:

    const std = @import("std");
    const expectFmt = std.testing.expectFmt;

    test "fmt.defaultchar.no-alignment" {

        // as of today the following test passes:
        try expectFmt("0x00ff", "0x{x:0>4}", .{255});

        // as of today the following test fails (returns "0x  ff" instead)
        try expectFmt("0x00ff", "0x{x:04}", .{255});
    }

* non breaking improvement of string formatting

* improved comment

* simplify the code a little

* small improvement around how  characters identified as valid are consumed

0 files changed, 0 insertions, 0 deletions