Update index.d.ts

[ericchanky]

This declaration cannot reflect the real type of the exported object. Wrapping it to interface so that can import { csv } from 'csvtojson with correct type

[coveralls]

Pull Request Test Coverage Report for Build 206

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 95.421%

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Totals
Change from base Build 203:	-0.3%
Covered Lines:	664
Relevant Lines:	680

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💛 - Coveralls