feat(useClipboard): support legacy copy

[lumdzeehol]

Description

Add legacy: boolean option.
It will fallback to legacy methods document.execCommand('copy') if ClipboardApi is not allowed, for scenes like hosting website with IP adress, especially during development.
Relate to #1276

Additional context

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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.

[sxzz]

IMHO, I don't think this feature is necessary. document.execCommand is supported in all modern browsers. https://caniuse.com/document-execcommand
Also, it will cause a breaking change if isSupported is changed.

If Clipboard API is not supported, then you can handle fallback by yourself.

[lumdzeehol]

IMHO, I don't think this feature is necessary. document.execCommand is supported in all modern browsers. https://caniuse.com/document-execcommand Also, it will cause a breaking change if isSupported is changed.

If Clipboard API is not supported, then you can handle fallback by yourself.

I think you're right. useClipboard should just focus on clipboardApi.
but what if i want a compatible use function to handle copy? is it necessary to create a use like useCopy or sth ? for that execCommand needs lots of template code everytime i code it.😢

[sxzz]

Yeah, I think it's a good idea, to combine Clipboard API and document.execCommand into one function called useCopy. WDYT @antfu

[antfu]

LGTM. Do you mind to send another PR to the document mention about this? Thanks