pmb.init: add _pmb_default APKBUILD key

A rudimentary implementation of the default suggestion mechanism proposed by @craftyguy and I at https://gitlab.com/postmarketOS/pmaports/-/issues/2718

With _pmb_default, packages can decide which package shown during _pmb_select will be chosen as default. If a package does not define _pmb_default or a package up for selection isn't found in _pmb_default then the package priority will determine the default as usual.