fix: add missing metadata to package.json files

[ZLNiao]

Summary

The published npm packages nivo@0.31.0 and @nivo/core@0.99.0 are missing several metadata fields in their package.json files.

Changes

• Root package.json: Added license (MIT), bugs, and homepage fields
• packages/core/package.json: Added bugs and homepage fields

Verification

• npm view nivo license bugs homepage --json — license and bugs return nothing
• npm view @nivo/core bugs homepage --json — both return nothing
• The repository already contains LICENSE.md, confirming the MIT license
• No dependencies, runtime code, or build configuration affected

[plouc]

There's no reason to touch the formatting, it makes reviewing more difficult, only metadata should be touched, nothing else.

[plouc]

Only metadata should be touched.