feat(computedAsync): introduce shadow option

[xiankq]

Description

Use shallowRef and mark the return value as readonly to conform to the computed ideas.

issues #2117

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[antfu]

shallowRef should be an options, just like:

vueuse/packages/core/useAsyncState/index.ts

Lines 88 to 91 in 2851c32

	shallow = true,
	throwError,
	} = options ?? {}
	const state = shallow ? shallowRef(initialState) : ref(initialState)

We need to make it default to false to avoid breaking change, and change to true in another PR aiming for the next major version

[xiankq]

hi，I've added shallow to the options, for which I also provided an update to the docs
code: 88de7b2
docs: b627a94

[xiankq]

I'm also preparing the next version : #2621