fix(type-defs): make interface ColorModeInstance extend Vue

[FelixGraf]

When using TypeScript and trying to create a watcher on the value property, the TypeScript compiler complains with:

TS2339: Property '$watch' does not exist on type 'ColorModeInstance'.

Code example:

import { Plugin } from '@nuxt/types'

const colorThemePlugin: Plugin = ({ $colorMode }) => {
  $colorMode.$watch('value', (val: string) => {
    console.log(val)
  })
}

export default colorThemePlugin

Making the interface ColorModeInstance extend Vue, the compiler does not complain.

[codecov]

Codecov Report

Merging #62 (2b42721) into master (1121daf) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #62   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           19        19           
  Branches         2         2           
=========================================
  Hits            19        19           

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