feat(common,developer,core): add 'form' to KMX+ 🎼

In trying to add tests for #12644 I realized that there isn't a KMX+ section for ``.

Now, in 99% of the cases, the implied fallback data from CLDR will be used, however, there is provision for custom forms.

Since at most one form is used per keyboard (if there's a hardware layout), we might only want to store that one form, i.e. just iso, etc.

A second question is whether we will want to store the *scan codes* (in the XML) or *vkeys) (what keyman actually uses).