Update three-core.d.ts to correct Color definition
Update three-core.d.ts to correct Color definition

lerpHSL is missing from the definition, but is [defined](https://github.com/mrdoob/three.js/blob/master/src/math/Color.js). 

The definition can also be improved by using 'self' to express that these functions are fluent, returning this rather than a new instance of Color.
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