Add Good First Issue tag, use it

[thewilsonator]

We should add a Good First Issue label and use it for issue that should have simple solutions e.g:

• BigInt multiplication of 0 by integral (non-BigInt) type creates "-0" (negative zero) #10565
• Incorrect %*s handling by formattedRead() #10598

[0xEAB]

AFAICT this label has been added by now; and it is in use: https://github.com/dlang/phobos/issues?q=is%3Aissue+label%3A%22Good+First+Issue%22

Can this be closed or is there something left to do?

[thewilsonator]

I added it and applied it to one issue. I still want to go through and see if there are any more that should have it so leave it open for now.