@actions/core: convert to ESM module

[danwkennedy]

Description

We're upgrading all the packages in this repo to ESM modules. @actions/core can go next since @actions/exec and @actions/http-client are already modules.

[Copilot]

Pull request overview

This PR converts the @actions/core package from CommonJS to ESM (ECMAScript Modules), introducing a breaking change that requires a major version bump to 3.0.0. CommonJS consumers will need to use dynamic import() instead of require().

Changes:

• Convert module system from CommonJS to ESM with appropriate TypeScript configuration
• Add .js extensions to all internal import statements as required by ESM
• Update package.json with "type": "module" and exports field configuration
• Upgrade dependencies to ESM-compatible versions (@actions/exec@3.0.0, @actions/http-client@4.0.0)

Reviewed changes

Copilot reviewed 13 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/core/package.json	Added "type": "module", exports field, bumped version to 3.0.0, updated dependencies to ESM versions
packages/core/package-lock.json	Updated lockfile with new dependency versions and their transitive dependencies
packages/core/tsconfig.json	Added module and moduleResolution settings for Node16 ESM support
packages/core/src/core.ts	Added .js extensions to all internal imports
packages/core/src/utils.ts	Added .js extensions to internal imports
packages/core/src/oidc-utils.ts	Added .js extension to core import
packages/core/src/file-command.ts	Added .js extension to utils import
packages/core/src/command.ts	Added .js extension to utils import
packages/core/tests/core.test.ts	Added .js extensions to internal imports in tests
packages/core/tests/summary.test.ts	Added .js extension to summary import
packages/core/tests/platform.test.ts	Added .js extension to core import
packages/core/tests/path-utils.test.ts	Added .js extension to path-utils import
packages/core/tests/command.test.ts	Added .js extension to command import
packages/core/RELEASES.md	Added 3.0.0 release notes documenting the ESM conversion and minor formatting improvements
jest.config.js	Added module name mappers for @actions/core, @actions/exec, and @actions/http-client/lib/interfaces, updated transformIgnorePatterns

Files not reviewed (1)

• packages/core/package-lock.json: Language not supported

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[schneidergithub]

Man this breaking change might create havoc and a support surge, good luck!