fix(runtime-core): patchChildren first in patchElement

[leaon4]

In mountElement of "renderer.ts", mountChildren will run before mountProps
https://github.com/vuejs/vue-next/blob/7ffa225aa334f0fd7da6ba30bee9109de3597643/packages/runtime-core/src/renderer.ts#L652-L657

But in patchElement, patchProps will run before patchChildren.
I think it is wrong.

for example:

test('patch innerHTML', () => {
  const root = document.createElement('div')
  render(
    h('div', { innerHTML: 'foo' }, 'hello'),
    root
  )
  const el = root.children[0]
  expect(el.innerHTML).toBe('foo') // props has higher priority

  render(
    h('div', { innerHTML: 'bar' }, 'world'),
    root
  )
  expect(el.innerHTML).toBe('bar') // error, received 'world', children has higher priority
})

another:

test('patch value for select', () => {
  const root = document.createElement('div')
  render(
    h('select', { value: 'foo' }, [
      h('option', { value: 'foo' }, 'foo'),
      h('option', { value: 'bar' }, 'bar')
    ]),
    root
  )
  const el = root.children[0] as HTMLSelectElement
  expect(el.value).toBe('foo') // correct, received "foo", because mountChildren first
  
  // bar to baz
  render(
    h('select', { value: 'baz' }, [
      h('option', { value: 'foo' }, 'foo'),
      h('option', { value: 'baz' }, 'baz')
    ]),
    root
  )
  expect(el.value).toBe('baz') // error, received "", because patchProps first
})

[yyx990803]

Good catch and thanks for the test cases!