types: add missing encryption option to ServerOptions

[AnonymoDGH]

Fixes #1503

Problem

The README's "Hello World server example" passes encryption: true to mc.createServer(...), and the option is used in several examples (examples/server_world, examples/server_channel, examples/server_custom_channel) and documented in HISTORY.md (renamed from encryption-enabled to encryption). However, encryption was missing from the ServerOptions interface in src/index.d.ts, so TypeScript users got an "Object literal may only specify known properties" error when following the documented example.

Fix

Added the missing optional encryption?: boolean property to ServerOptions:

export interface ServerOptions {
    ...
    'online-mode'?: boolean
    /** Whether to enable encryption (handshake + encrypted transport). Used together with online-mode. */
    encryption?: boolean
    ...
}

Verified

• tsc --noEmit --skipLibCheck src/index.d.ts passes with the change.
• The property matches the documented README example and the existing examples that pass encryption to createServer.

[AnonymoDGH]

CI note: the single test (1.21.5) failure is a transient network error in the before all hook (FetchError: request to https://launchermeta.mojang.com/mc/game/version_manifest.json failed) — 227 tests passed and the failure occurs before any test logic runs. It is unrelated to this change, which only adds a property to src/index.d.ts (no runtime code). Pushed an empty commit to re-trigger CI.