feat: migrate to ESM and upgrade all dependencies

[mgerlach]

Summary

• Migrates the project from CommonJS to ESM to support @actions/core v3, which is ESM-only
• Upgrades all pending dependabot dependency bumps in a single PR
• All 4 tests pass locally on Node 24

Breaking change: @actions/core v3 is ESM-only

To support it, the following migration was required:

• package.json: added "type": "module", updated test script to use node --experimental-vm-modules for Jest ESM support
• tsconfig.json: switched to module/moduleResolution: NodeNext, added isolatedModules: true (required by ts-jest with NodeNext)
• jest.config.js: enabled useESM in ts-jest transform, added extensionsToTreatAsEsm: ['.ts'] and moduleNameMapper to resolve .js imports to .ts source files
• src/main.ts: updated relative import to ./asdf.js (required by ESM)
• __tests__/asdf.test.ts: replaced __dirname with import.meta.url-based equivalent, updated import to .js extension
• __tests__/main.test.ts: updated import to .js extension
• .eslintrc.json: disabled import/no-unresolved (produces false positives for the .js→.ts ESM convention)

Dependency upgrades

Resolves all pending dependabot PRs:

Package	From	To	PR
@actions/core	2.0.2	3.0.1	#507
eslint-plugin-jest	29.12.1	29.15.2	#508
@types/node	25.0.10	25.9.1	#509
@typescript-eslint/eslint-plugin	8.53.1	8.59.4	#510
actions/upload-artifact	v6	v7	#511
@typescript-eslint/parser	8.53.1	8.59.4	#505

Test plan

• npm run build — TypeScript compiles without errors
• npm run format — no formatting changes needed
• npm run lint — no lint errors
• npm run package — ncc bundles successfully (auto-detected ESM output)
• npm test — all 4 tests pass (2 suites)

Closes #507, #508, #509, #510, #511, #505

🤖 Generated with Claude Code

[mgerlach]

I am not a total NodeJs/JavaScript/TypeScript expert, so I hope Claude hasn't produced too much slop. I challenged some of the suspicious looking configs, so Claude attempted a few things to reduce the size of the change, but did not succeed. So I did a bit of research regarding some of the changes myself, it looks like Claude is right as is.

This PR should be independent from #513.

[endorama]

Thanks! I'll break this down into smaller and more focused PRs. The changelog is easier to follow that way, considering this is something people install in their CI.

I should be able to push on this branch, no action should be needed from your side.

[mgerlach]

Thanks! I'll break this down into smaller and more focused PRs. The changelog is easier to follow that way, considering this is something people install in their CI.

I should be able to push on this branch, no action should be needed from your side.

Hi @endorama, do you need help with splitting up the PR?