feat(types): update to Typescript 3.9 (#1106)

test-dts/index.d.ts

@@ -7,3 +7,7 @@
 export * from '@vue/runtime-dom'
 
 export function describe(_name: string, _fn: () => void): void
+
+export function expectType<T>(value: T): void
+export function expectError<T>(value: T): void
+export function expectAssignable<T, T2 extends T = T>(value: T2): void