feat: add typings for reporterOptions

[AriPerkkio]

Checklist

• npm test, tests passing
• npm run test:snap (to update the snapshot)
• tests and/or benchmarks are included
• documentation is changed or added

Adds typings for reporterOptions implemented in #423. Demo at TS Playground.

These typings are not 100% exact, as reporter is neither strictly typed to known values only.
Typings could be improved by utilizing the @types/istanbul-reporters package if wanted:

import type { ReportOptions } from 'istanbul-reports'

export interface Options {
  reporter?: (keyof ReportOptions)[]
  reporterOptions?: {
    [Reporter in keyof ReportOptions]: Partial<ReportOptions[Reporter]>
  }
}

[bcoe]

@AriPerkkio thanks!