Add south_korea alias, korea tags for North Korea

[Harshcombo50548]

Add south_korea alias, korea tags for North Korea

The 🇰🇷 flag has no alias containing either word of its name, so in practice it's reachable
only by typing kr exactly.

Current data:

🇰🇷  aliases: ["kr"]            tags: ["korea"]
🇰🇵  aliases: ["north_korea"]   tags: []

Emoji.find_by_alias("south_korea") returns nil, and alias-substring search misses it from
both directions:

Search	Matches today	🇰🇷
south	south_africa, south_sudan, south_georgia_south_sandwich_islands, french_southern_territories	❌ no alias contains "south"
korea	north_korea	❌ "korea" is a tag, not an alias

South Korea does carry korea as a tag, so a tag-aware consumer can find it — but alias lookup
never sees tags, and pickers that do search tags rank those hits below alias matches, so 🇰🇵 wins
the "korea" query. 🇰🇵, meanwhile, has no tags at all, so it isn't findable by tag search either.

Change

🇰🇷  aliases: ["kr", "south_korea"]   tags: ["korea", "south"]
🇰🇵  aliases: ["north_korea"]         tags: ["korea", "north"]

Appending the full country name after a short canonical alias follows the existing
eu/european_union shape. North Korea gets korea/north tags to close the same gap from
the tag side, without touching its aliases.

After the change, an alias search for south returns 5 flags including South Korea, and
korea returns both Koreas.

Notes

• Existing aliases stay in first position. Character#name is aliases.first, so
  find_by_unicode("🇰🇷").name still returns "kr" and :kr: / :north_korea: are
  unaffected. Nothing is renamed or removed, and North Korea's alias list is untouched.
• No collisions. south_korea was unused across all 1870 entries, so the "emojis have
  valid names" duplicate assertion still holds. (For anyone considering the same treatment for
  the other ISO-2 flags: japan is already taken by 🗾 and turkey by 🦃.)
• Survives regeneration. db/dump.rb copies existing_emoji.aliases and
  existing_emoji.tags verbatim, so rake db:dump won't clobber these. Both flag sequences
  match on the first find_by_unicode lookup and appear exactly once in
  vendor/unicode-emoji-test.txt, so the seen_existing de-dup path can't regenerate a fresh
  alias for them.
• The diff is 4 added lines in db/emoji.json, hand-edited to match the file's generated
  leading-comma formatting.

[Harshcombo50548]

@mislav This repo Pull requests seem stale? Is it still being maintained?

Side Note: The south_korea alias is the only thing that actually matters here.