Bump @actions/core from 1.10.1 to 3.0.0 and @actions/exec from 1.1.1 to 3.0.0

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.

jest.config.js

@@ -1,11 +0,0 @@
-module.exports = {
-  clearMocks: true,
-  moduleFileExtensions: ['js', 'ts'],
-  testEnvironment: 'node',
-  testMatch: ['**/*.test.ts'],
-  testRunner: 'jest-circus/runner',
-  transform: {
-    '^.+\\.ts$': 'ts-jest'
-  },
-  verbose: true
-}