feat: expose blockchain id constants

[wyf027]

Summary

• Exposes the existing blockchain ID constants as static properties on Web3Data.
• Updates the built browser bundle so CDN/browser consumers can access the same constants.

Fix/Approach

• Reuses the constants already defined in src/constants.js.
• Assigns each blockchain ID constant onto the exported Web3Data class, matching the issue example (Web3Data.BLOCKCHAIN_ID_ETHEREUM_MAINNET).
• Adds an AVA regression test covering all currently defined blockchain ID constants.

Verification

• RED: npx ava test/web3data.test.js --match='Web3data exposes blockchain id constants as public class properties' -- --mode replay failed before the implementation with undefined for Web3Data.BLOCKCHAIN_ID_ETHEREUM_MAINNET.
• GREEN: npx ava test/web3data.test.js --match='Web3data exposes blockchain id constants as public class properties' -- --mode replay passed after the implementation: 1 test passed.
• npm test passed: 181 tests passed, 52 tests skipped.
• npm run build passed and regenerated dist/web3data.min.js.
• git diff --check passed.

Notes

• CONTRIBUTING.md asks contributors to sign a CLA. I cannot sign legal documents on behalf of anyone, so a CLA check may remain pending until an authorized contributor handles it.

Related issue

Fixes #68