Add GitHub Actions workflow with ESLint and Jest

[devin-ai-integration]

Add GitHub Actions workflow with ESLint and Jest

Summary

This PR introduces a modern CI/CD pipeline using GitHub Actions to replace the existing Travis CI setup. The workflow includes code linting with ESLint and comprehensive testing across multiple Node.js versions.

Key changes:

• Added ESLint configuration with basic JavaScript linting rules
• Created GitHub Actions workflow with separate lint and test jobs
• Updated Node.js versions from 10/11/12 (Travis) to 18.x/20.x/22.x (GitHub Actions)
• Added lint scripts to package.json for local development

The ESLint configuration uses warnings instead of errors for existing code patterns to avoid breaking the build while still providing code quality feedback.

Review & Testing Checklist for Human

• Verify GitHub Actions workflow runs successfully - Check that both lint and test jobs pass in this PR
• Review ESLint configuration appropriateness - The rules are currently set to "warn" to accommodate existing code; verify this aligns with project standards
• Confirm Node.js version strategy - Upgraded from Node 10/11/12 to 18.x/20.x/22.x per package.json engines requirement (>=18)
• Test lint command locally - Run npm run lint to ensure ESLint warnings are acceptable and don't indicate real issues

Notes

• ESLint found 9 existing code quality issues (unnecessary escapes, empty blocks, prototype method usage) that are now reported as warnings
• The workflow replaces Travis CI but maintains the same core functionality (test + build)
• All 72 existing Jest tests continue to pass

Link to Devin run: https://app.devin.ai/sessions/f64ceefe350e4a42bf304bfacf32c3f7
Requested by: jason.julian

[devin-ai-integration]

Original prompt from cedric@coingecko.com

In the cryptoformat repo, look into the file https://github.com/coingecko/cryptoformat/blob/master/src/index.js. Set up unit tests for this module to test all happy path and edge cases for each function using Jest

[devin-ai-integration]

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[devin-ai-integration]

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