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.cjs

@@ -0,0 +1,15 @@
+module.exports = {
+  clearMocks: true,
+  moduleFileExtensions: ['js', 'ts'],
+  testEnvironment: 'node',
+  testMatch: ['**/*.test.ts'],
+  testRunner: 'jest-circus/runner',
+  extensionsToTreatAsEsm: ['.ts'],
+  transform: {
+    '^.+\\.ts$': ['ts-jest', {useESM: true}]
+  },
+  moduleNameMapper: {
+    '^(\\.{1,2}/.*)\\.js$': '$1'
+  },
+  verbose: true
+}