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/cosmokit/src/array.ts

@@ -1,4 +1,4 @@
-import { isNullable } from './misc'
+import { isNullable } from './misc.ts'
 
 /** Return true when every item in `array2` is present in `array1`. */
 export function contain(array1: readonly any[], array2: readonly any[]) {