feat(useColorMode): add option to return auto mode

[dfreier]

Description

Some applications need to know if the preferred color mode was used, e.g. when showing a dropdown menu with the three options light, dark or system. With the current implementation, there is no way to figure out if the user selected auto mode. That means it's not possible to highlight the system option in the dropdown as active.

I added an option emitAuto which defaults to false. When enabled, it includes the auto mode in the returned state variable.

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.

[antfu]

Instead of "omit" and defaulting to true, I think it's better to have it as opt-in, like "emitAuto" and defaulting to false

[dfreier]

Makes sense, feels more natural 👍 I pushed another commit to change it.