fix(anvil): use the 50 character name limit on 1.17+

[AnonymoDGH]

Fixes #3945

Problem

anvil.combine and anvil.rename rejected any custom name longer than 35 characters on every version. Since 1.17, the vanilla anvil name limit is 50 characters, so valid 36–50 character names were incorrectly refused with Name is too long.

Fix

minecraft-data already ships a feature flag for exactly this change — anvilNameLengthIsFifty (1.17 → latest, added in PrismarineJS/minecraft-data#1207). This computes the limit once at plugin injection:

const maxNameLength = bot.supportFeature('anvilNameLengthIsFifty') ? 50 : 35

and uses it in both combine and rename. Behavior on ≤1.16.5 is unchanged (35).

Verified

• supportFeature('anvilNameLengthIsFifty') resolves to false on 1.8/1.12.2/1.16.5 and true on 1.17/1.18.2/1.20.1/1.21.1 with the installed minecraft-data (3.113.2, within the existing ^3.108.0 range — no dependency bump needed).
• bot.supportFeature is assigned in loader.js before plugins are injected, so it is available at inject time.
• standard lint passes on the changed file.

(The anvil integration tests in test/externalTests/anvil.js are excluded from CI and require a live server, so no test changes are included.)