Use explicit ts specifiers for declarations

Restore explicit .ts relative specifiers in source and enable rewriteRelativeImportExtensions so emitted JS uses .js while declarations keep explicit .ts specifiers. Add a NodeNext declaration-consumer gate to prevent extensionless declaration regressions.

vendor/cordis/src/utils.ts

@@ -1,5 +1,5 @@
 import { defineProperty } from 'cosmokit'
-import type { Context, Service } from '.'
+import type { Context, Service } from './index.ts'
 
 /** Ordered collection of disposable values with O(1) deletion by value. */
 export class DisposableList<T extends WeakKey> {