[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`

cleanup.js

@@ -0,0 +1,10 @@
+const core = require('@actions/core')
+const { execSync } = require('child_process')
+
+try {
+    // Kill the started SSH agent
+    console.log('Stopping SSH agent')
+    execSync('kill ${SSH_AGENT_PID}', { stdio: 'inherit' })
+} catch (error) {
+    core.setFailed(error.message)
+}