Both packages moved to ESM-only in v3, so this migrates the project
from CommonJS to ESM: adds "type": "module" to package.json, switches
tsconfig to module/moduleResolution "nodenext", adds .js extensions to
all relative imports, and reconfigures Jest for ESM support.
Upgrade @actions/core (1.x→2.x), @actions/exec (1.x→2.x), and
@actions/github (6.0.0→8.0.1) to resolve undici <6.23.0 unbounded
decompression chain vulnerability. Add skipLibCheck to tsconfig.json
for compatibility with @octokit/core@7 type declarations.
* Rework change detection via `git diff`
Previous implementation performed simple diff between two versions. New implementation fetches on demand more commits to have the merge base between two branches. Now it will detect only changes introduced by branch that was pushed, instead of mixing with changes introduced meanwhile on the base branch.
