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/service.ts

@@ -1,6 +1,6 @@
 import { defineProperty } from 'cosmokit'
-import { Context } from './context'
-import { createCallable, joinPrototype, symbols, Tracker } from './utils'
+import { Context } from './context.ts'
+import { createCallable, joinPrototype, symbols, Tracker } from './utils.ts'
 
 /**
  * Base class for services that expose a named API on `ctx`.