Add @msftbot automation

[RussKie]

For the most part this is a stripped down copy from https://github.com/dotnet/winforms/blob/main/.github/fabricbot.json which I authored over the years. I aligned the labels in this repo to match.

[danmoseley]

seems reasonable. do we need to do anything to make fabricbot start reading this?

later we'll probably want to look at runtime/aspnetcore ones for any other goodness.

[joperezr]

Thanks for adding this @RussKie. I had a couple of comments and have addressed them here: #3988

[RussKie]

do we need to do anything to make fabricbot start reading this?

No, once the config is in, the bot starts using it instead of a config stored in its database somewhere.