fix(useColorMode): resolve auto to preferred mode internally

[dfreier]

Description

I recently added the emitAuto option to useColorMode (#1627).
Now I discovered a bug with that. The HTML class attribute is now being updated with auto class, which is wrong. Even if the emitAuto option is enabled and the value auto is emitted to the user of the composable, we still need to resolve that in either light or dark internally.

Additional context

How to reproduce the bug:

1. Set your system into light mode
2. Visit https://vueuse.org/core/usecolormode/
3. The page should render in light mode
4. Set your system into dark mode
5. The page should render in dark mode
6. Press the green button until it displays Auto again
7. The page renders in light mode, which is wrong

With this fix, the page correctly renders in dark for step 7).
Note: Fix only works with this other fix for useMediaQuery #1765

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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