[FEATURE] Post step for cleaning up the SSH agent

* add `cleanup.js` for stopping the ssh-agent after the job is done
* add `scripts/build.js` to make it easier to build the files in `dist/`
* update README
* remove meaningless dependency `child_process` (its a node core package)
* update `@actions/core` to `^1.2.4`

scripts/build.js

@@ -0,0 +1,35 @@
+const { execSync } = require('child_process')
+const path = require('path')
+const fs = require('fs')
+
+const buildDir = path.join(process.cwd(), 'build')
+const distDir = path.join(process.cwd(), 'dist')
+
+const buildIndexJs = path.join(buildDir, 'index.js')
+const distIndexJs = path.join(distDir, 'index.js')
+const distCleanupJs = path.join(distDir, 'cleanup.js')
+
+if (!fs.existsSync(buildDir)) {
+    fs.mkdirSync(buildDir)
+}
+
+// Build the main index.js file
+console.log('Building index.js...')
+execSync(`./node_modules/.bin/ncc build index.js -q -o ${buildDir}`)
+if (fs.existsSync(distIndexJs)) {
+    fs.unlinkSync(distIndexJs)
+}
+fs.renameSync(buildIndexJs, distIndexJs)
+
+// Build the cleanup.js file
+console.log('Building cleanup.js...')
+execSync(`./node_modules/.bin/ncc build cleanup.js -q -o ${buildDir}`)
+if (fs.existsSync(distCleanupJs)) {
+    fs.unlinkSync(distCleanupJs)
+}
+fs.renameSync(buildIndexJs, distCleanupJs)
+
+console.log('Cleaning up...')
+fs.rmdirSync(buildDir)
+
+console.log('Done')