refactor(watch): adjsut watch API behavior

BREAKING CHANGE: `watch` behavior has been adjusted.

    - When using the `watch(source, callback, options?)` signature, the
      callback now fires lazily by default (consistent with 2.x
      behavior).

      Note that the `watch(effect, options?)` signature is still eager,
      since it must invoke the `effect` immediately to collect
      dependencies.

    - The `lazy` option has been replaced by the opposite `immediate`
      option, which defaults to `false`. (It's ignored when using the
      effect signature)

    - Due to the above changes, the `watch` option in Options API now
      behaves exactly the same as 2.x.

    - When using the effect signature or `{ immediate: true }`, the
      intital execution is now performed synchronously instead of
      deferred until the component is mounted. This is necessary for
      certain use cases to work properly with `async setup()` and
      Suspense.

      The side effect of this is the immediate watcher invocation will
      no longer have access to the mounted DOM. However, the watcher can
      be initiated inside `onMounted` to retain previous behavior.

test-dts/watch.test-d.ts

@@ -25,22 +25,39 @@ watch([source, source2, source3] as const, (values, oldValues) => {
 })
 
 // lazy watcher will have consistent types for oldValue.
+watch(source, (value, oldValue) => {
+  expectType<string>(value)
+  expectType<string>(oldValue)
+})
+
+watch([source, source2, source3], (values, oldValues) => {
+  expectType<(string | number)[]>(values)
+  expectType<(string | number)[]>(oldValues)
+})
+
+// const array
+watch([source, source2, source3] as const, (values, oldValues) => {
+  expectType<Readonly<[string, string, number]>>(values)
+  expectType<Readonly<[string, string, number]>>(oldValues)
+})
+
+// source + immediate: true
 watch(
   source,
   (value, oldValue) => {
     expectType<string>(value)
-    expectType<string>(oldValue)
+    expectType<string | undefined>(oldValue)
   },
-  { lazy: true }
+  { immediate: true }
 )
 
 watch(
   [source, source2, source3],
   (values, oldValues) => {
     expectType<(string | number)[]>(values)
-    expectType<(string | number)[]>(oldValues)
+    expectType<(string | number | undefined)[]>(oldValues)
   },
-  { lazy: true }
+  { immediate: true }
 )
 
 // const array
@@ -48,7 +65,9 @@ watch(
   [source, source2, source3] as const,
   (values, oldValues) => {
     expectType<Readonly<[string, string, number]>>(values)
-    expectType<Readonly<[string, string, number]>>(oldValues)
+    expectType<
+      Readonly<[string | undefined, string | undefined, number | undefined]>
+    >(oldValues)
   },
-  { lazy: true }
+  { immediate: true }
 )