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.

pmb/config/__init__.py

@@ -238,6 +238,9 @@ apkbuild_package_attributes = {
     # additional configuration options in "pmbootstrap init" that allow
     # selecting alternative providers for a virtual APK package.
     "_pmb_select": {"array": True},
+    # postmarketos-base and UI meta-packages can define the default package
+    # to select during "_pmb_select".
+    "_pmb_default": {"array": True},
 }
 
 # Variables in APKBUILD files that get parsed