refactor(set_theory/game/nim): remove `nim` namespace (#15366)
A lot of theorems that already had `nim` in the name were also in the `pgame.nim` namespace. We remove the `nim` namespace entirely, and rename the few theorems that didn't have `nim` in the name:
- `non_zero_first_wins` → `nim_fuzzy_zero_of_ne_zero`
- `add_equiv_zero_iff_eq` → `nim_add_equiv_zero_iff`
- `add_fuzzy_zero_iff_ne` → `nim_add_fuzzy_zero_iff`
- `equiv_iff_eq` → `nim_equiv_iff_eq`
- `grundy_value` → `nim_grundy_value`
Further, we move `nim` itself into the `pgame` namespace.
