Formatting `BigInt` with `%x` inserts digit separators

[Bolpat]

It seems BigInt hex formatting inserts _ after 8 digits. That is unexpected and cannot be shut off. Digit separators are supported by formatting via ,, so in principle, when needed, they can be requested, but that doesn’t work either.

void main()
{
    import std.bigint;
    import std.format;
    auto x = (BigInt(1L) << 65) + 5;
    assert(format("%x", x) == "20000000000000005"); // fails
    assert(format("%,4?x", '_', x) == "2_0000_0000_0000_0005"); // fails
}

[Prashant-900]

I took a look at this issue and it seems the value is hardcoded and can be fixed with a small change.
I’m new to contributing and would be happy to open a PR for this if no one else is already working on it.