Fix type imports

[arjunyel]

https://arethetypeswrong.github.io/?p=decimal.js-light%402.5.1

Without this change I get the error:

import { Decimal } from "decimal.js-light";
         ^

SyntaxError: The requested module 'decimal.js-light' does not provide an export named 'Decimal'

Fix types by copying decimal.d.ts to decimal.d.mts.

Test with: npx @arethetypeswrong/cli@latest --pack .