refactor(actions): simplify action path structure

- Relocates action files to a flat directory structure
- Updates references to the new paths for better organization

with-post-step/main.js

@@ -0,0 +1,18 @@
+const { spawn } = require("child_process");
+const { appendFileSync } = require("fs");
+const { EOL } = require("os");
+
+function run(cmd) {
+  const proc = spawn(cmd, { stdio: "inherit", shell: true });
+  proc.on("exit", code => process.exitCode = code);
+}
+
+const key = process.env.INPUT_KEY.toUpperCase();
+if (process.env[`STATE_${key}`] !== undefined) {
+  // ---------- POST ----------
+  run(process.env.INPUT_POST);
+} else {
+  // ---------- MAIN ----------
+  appendFileSync(process.env.GITHUB_STATE, `${key}=true${EOL}`);
+  run(process.env.INPUT_MAIN);
+}