feat(useMax): new function

[FrankFang]

Description

import { useMaximum } from '@vueuse/math'

const max1 = ref(10)
const max2 = ref(20)
const result = useMaximum(max1, max2)
// result.value is 20
max2.value = 50
// result.value is 50 now

Additional context

#1812

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.