feat(developer): Discussion on PUA and hacked keyboards

@dyacob had a suggestion that we should maybe recommend PUA keyboards to have the ID with the end of the ID be `pua`. For example, `gff_sbs` (which is PUA-encoded) should maybe have been reserved for when the script gets into Unicode. The PUA-encoded keyboard should maybe have been called `gff_sbs_pua` and then when the script gets added to Unicode it would change back to the ID without `_pua`. It's too late for this keyboard, but I like that idea.

Then, I've heard complaints about Keyman (me?) not accepting hacked encoded keyboards. It makes it difficult to use a RTL script when we don't let them use Arabic codepoints. **I'm still opposed**, but there are some who think we should allow it. If we do decide to allow it, I would suggest:
- Adding another stream called `custom` or `hacked`
- Allow people to reuse existing Unicode codepoints (for example Arabic codepoints)
- Any keyboard in this stream would have a warning at the top of the keyboard page saying "This keyboard is using hacked codepoints and will only work with the included font"
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