Upgrade the module to ESM and bump dependencies

[danwkennedy]

Description

Before picking up the update to support direct uploads, we need to convert the package to ESM.

This PR does the following:

• Upgrades the package to ESM
• Upgrade a bunch of dependencies (so we pick up ESM support for those as well)
• Notably, we're on a higher version of eslint and jest
• Updates our CI to use Node v24. That was missed in earlier PRs

[Copilot]

Pull request overview

This pull request upgrades the upload-artifact action from CommonJS to ECMAScript Modules (ESM) and bumps all major dependencies to their latest versions. The migration is comprehensive and includes updating TypeScript configuration, test infrastructure, linting setup, and all source files to follow ESM conventions.

Changes:

• Migrated module system from CommonJS to ESM with updated TypeScript configuration
• Updated all dependencies to latest major versions (Jest 30, ESLint 9, TypeScript 5.3, etc.)
• Converted all test files to use ESM mocking patterns with jest.unstable_mockModule
• Added .js extensions to all relative imports as required by ESM
• Migrated from legacy ESLint configuration to flat config format
• Converted Jest configuration from .js to .ts with ESM-specific settings

Reviewed changes

Copilot reviewed 15 out of 19 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tsconfig.json	Updated compiler options for ESM: NodeNext module resolution, ES2022 target, removed unused options
tsconfig.eslint.json	New ESLint-specific TypeScript config to include test files and root-level TS files
package.json	Added "type": "module", updated test command for ESM, bumped all dependencies to latest versions
jest.config.ts	New ESM-compatible Jest config with ts-jest ESM support and module name mapping
jest.config.js	Removed old CommonJS Jest config
eslint.config.mjs	New ESLint flat config format replacing .eslintrc.json
.eslintrc.json	Removed legacy ESLint configuration
src/upload/*.ts	Added .js extensions to all relative imports for ESM compatibility
src/merge/*.ts	Added .js extensions to all relative imports for ESM compatibility
tests/*.test.ts	Migrated to ESM test patterns using jest.unstable_mockModule and dynamic imports, changed 'it' to 'test', added __dirname replacement for ESM
.gitignore	Added .DS_Store for Mac development environments

Comments suppressed due to low confidence (1)

eslint.config.mjs:30

• The ESLint configuration references 'import/*' rules (lines 27-30) but 'eslint-plugin-import' has been removed from devDependencies. If these rules are not provided by 'eslint-plugin-github', this will cause ESLint to fail with "Definition for rule 'import/no-namespace' was not found" errors. Either add 'eslint-plugin-import' back to devDependencies and configure it in the ESLint config, or remove these rule configurations if they're no longer needed.

      'import/no-namespace': 'off',
      'import/no-commonjs': 'off',
      'import/named': 'off',
      'import/no-unresolved': 'off',

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[philip-gai]

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