Fix Dragon's Fire targeting issue / refactor

[oscscull]

Was testing an implementation of Duskmourn's "Monstrous Emergence". I based the implementation on Dragon's Fire.
During that testing, realised the creature you "choose" fails if they have shroud or protection, because it targets, despite saying "choose". Confirmed it has the same issue in the original implementation, so this PR is to fix that bug. I based the fix on code from Sculpted Sunburst and Final Showdown.

Also, there was/is a warn based on using this enum with HAND/BATTLEFIELD/NONE. The NONE case seems to be not useful (and comes with a warning on the switch statement) as far as I can see, so I replaced it with null. Perhaps there's some need for it I haven't understood.

[jeffwadsworth]

[[Dragon's Fire]]

[github-actions[bot]]

Dragon's Fire - (Gatherer) (Scryfall) (EDHREC)

{1}{R} Instant As an additional cost to cast this spell, you may reveal a Dragon card from your hand or choose a Dragon you control. Dragon's Fire deals 3 damage to target creature or planeswalker. If you revealed a Dragon card or chose a Dragon as you cast this spell, Dragon's Fire deals damage equal to the power of that card or creature instead.

[ssk97]

This looks good to me, thanks for the fix!