Feature request: Add hook for transforming minisearch index data

[GulomovCreative]

Is your feature request related to a problem? Please describe.

At this moment, we have no way to somehow influence the fields being added to the local search index, except to write new solution. It would be more convenient if it was possible to change and/or add fields in a hook similar to transformData

Describe the solution you'd like

export default {
  transformPageIndex(index, { documentId }) {
    index.newField = 'someValue'
    index.category = getCategory(documentId)
  }

  // or

  transformPageIndex(index, { documentId }) {
    return {
      newField: 'someValue',
      category: getCategory(documentId)
    }
  }
}

Describe alternatives you've considered

No response

Additional context

No response

Validations

• [X] Follow our Code of Conduct
• [X] Read the docs.
• [X] Read the Contributing Guidelines.
• [X] Check that there isn't already an issue that asks for the same feature to avoid creating a duplicate.

[brc-dd]

Should be possible using something like this (ignore ts errors, if any):

import { defineConfig } from 'vitepress'

export default defineConfig({
  themeConfig: {
    search: {
      provider: 'local',
      options: {
        miniSearch: {
          options: {
            fields: ['title', 'titles', 'text', 'category'],
            storeFields: ['title', 'titles', 'category'],
            extractField(document, fieldName) {
              if (fieldName === 'category') return getCategory(document)
              return document[fieldName]
            }
          },
          searchOptions: {
            /* ... */
          }
        }
      }
    }
  }
})