Fix better_villagers worker rooms never generating (Forge/1.20.1)

[0-bingci]

Fixes #6

The Create-themed worker rooms (andesite/brass/copper) shipped in 1.5.7 could never
generate. This PR contains the JSON-side fixes:

- Corrected the three structure `location` paths in the better_villagers pools:
  `addons/better_villager/...` -> `addons/better_villagers/...` (the actual NBT location)
- Added the three worker rooms to `stoneholm:point_of_interest` and
  `stoneholm:abandoned_point_of_interest` at weight 30 (same tier as copper_shop /
  libraries), since nothing referenced the addon pools before

Not covered here (needs re-saving structures in-game, see the issue):
- `stoneholm:villager` / `stoneholm:iron_golm` misspellings inside bedroom NBT jigsaws

Tested locally on 1.20.1 Forge: with these changes the worker rooms generate correctly,
including chests and their loot tables.