feat(vitepress-twoslash): improve UX for option customization

[9romise]

Description

This PR makes it more convenient for users to modify existing themes.

Important

One thing to note: I've changed the theme option to themes to align with upstream naming. This is a breaking change, so if you prefer, I can move this adjustment to a separate PR. Just let me know!

Linked Issues

Related: vueuse/vueuse#4983

Additional context

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[antfu]

Is it possible to keep backward compatibilty and mark it as deprecated?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.41%. Comparing base (d05f39b) to head (b99c167).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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  Coverage   95.41%   95.41%           
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  Files          89       89           
  Lines        7576     7576           
  Branches     1593     1593           
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  Hits         7229     7229           
  Misses        341      341           
  Partials        6        6           

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[9romise]

The option type (FloatingVueConfig) is any, and it's from upstream. Maybe I should also contribute to it for the better type.