Add customColors to typescript definition for PrettyOptions

[Hufschmidt]

See https://github.com/pinojs/pino-pretty/issues/439

Also contains minor cleanup of type test-file.

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This assumes index.d.ts is managed manually and not autogenerated by some process as I didn't see any related script in package.json.

Also I've not worked with TSD so far, so I only added the missing example values for customLevels and customColors parameters to the options. A quick inverse check confirmed tests working as expected as they failed without adding customColors to the type definition. :+1:

[coveralls]

Pull Request Test Coverage Report for Build 5599626822

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 5455898307:	0.0%
Covered Lines:	410
Relevant Lines:	410

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💛 - Coveralls