types: fix + test inject API typing

ref: #2052

test-dts/inject.test-d.ts

@@ -0,0 +1,15 @@
+import { provide, inject, InjectionKey, expectType } from './index'
+
+const key: InjectionKey<number> = Symbol()
+
+provide(key, 1)
+// @ts-expect-error
+provide(key, 'foo')
+
+expectType<number | undefined>(inject(key))
+expectType<number>(inject(key, 1))
+expectType<number>(inject(key, () => 1, true /* treatDefaultAsFactory */))
+
+expectType<() => number>(inject('foo', () => 1))
+expectType<() => number>(inject('foo', () => 1, false))
+expectType<number>(inject('foo', () => 1, true))