Types: Export DecimationAlgorithm as const

[kurkle]

Fix #8993

This only affects the typescript compiler output of a typescript project consuming these types.
With declare, the DecimationAlgorithm object is assumed to be found in chart.esm.js, and its not, because we only have that in the '.d.ts'.

As said in https://stackoverflow.com/a/50568865/10359775 the const enum is however replaced with the string values at compile time, so the end result does not try to use DecimationAlgorithm from the chart.esm.js, but uses the literal strings instead.

More information: https://www.typescriptlang.org/docs/handbook/enums.html#enums-at-runtime