feat(core)!: always apply default extractor

[antfu]

This PR introduced extractorDefault option and make extractorSplit always applied. So when providing extractors, it no longer overrides the default, making the usage simpler. It means to use extractorPug or extractorSvelte, you no longer need to specify extractorSplit along side.

To force override the default extractor, users or presets could provide extractorDefault option:

// unocss.config.ts
import { defineConfig } from '@unocss/core'

export default defineConfig({
  extractors: [
    // your extractors
  ],
  // disable the default extractor
  extractorDefault: false,
  // override the default extractor with your own
  extractorDefault: myExtractor,
})

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