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/events.ts

@@ -1,7 +1,7 @@
 import { defineProperty, Promisify } from 'cosmokit'
-import { Context } from './context'
-import { Fiber, FiberState } from './fiber'
-import { DisposableList, symbols } from './utils'
+import { Context } from './context.ts'
+import { Fiber, FiberState } from './fiber.ts'
+import { DisposableList, symbols } from './utils.ts'
 
 /** Return whether an event result should stop a bail-style dispatch. */
 export function isBailed(value: any) {
@@ -25,7 +25,7 @@ export type ThisType<F> = F extends (this: infer T, ...args: any) => any ? T : n
  */
 export type DispatchMode = 'emit' | 'parallel' | 'serial' | 'bail' | 'waterfall'
 
-declare module './context' {
+declare module './context.ts' {
   export interface Context {
     /* eslint-disable max-len */
     parallel<K extends keyof Events>(name: K, ...args: Parameters<Events[K]>): Promise<void>