Fix import types

[arjunyel]

https://arethetypeswrong.github.io/?p=bignumber.js%409.1.2

Fix imports and types by copying bignumber.d.ts to bignumber.d.mts.

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

[AdamVig]

I'm trying to migrate from CommonJS to ES Modules and this issue is getting in the way. I'm seeing errors like the following on tsc:

src/index.ts(20,37): error TS2339: Property 'clone' does not exist on type 'typeof import("node_modules/bignumber.js/bignumber")'.
src/index.ts(22,28): error TS2339: Property 'ROUND_HALF_EVEN' does not exist on type 'typeof import("node_modules/bignumber.js/bignumber")'.

Manually applying the changes from this pull request in node_modules/bignumber.js fixes the build. With those changes, npx @arethetypeswrong/cli@latest --pack node_modules/bignumber.js passes.

@MikeMcl Any hesitation about merging this? I'm happy to help in any way I can.