cli: add promisify spawn

cli.ts

@@ -1,4 +1,4 @@
-import { SpawnSyncOptionsWithBufferEncoding, SpawnSyncReturns, spawnSync as rawSpawnSync } from "child_process";
+import { ChildProcess, SpawnSyncOptionsWithBufferEncoding, SpawnSyncReturns, spawnSync as rawSpawnSync } from "child_process";
 
 export function check(result: SpawnSyncReturns<Buffer | string>) {
     if (result.error)
@@ -16,3 +16,13 @@ export function checkedSpawnSync(
 ): SpawnSyncReturns<Buffer | string> {
     return check(rawSpawnSync(command, args, options));
 }
+
+export function promisifySpawn<T extends ChildProcess>(p: T) {
+    return new Promise<T>((resolve, reject) => {
+        p.on("error", err => reject(err));
+        p.on("close", number => {
+            if (number != 0) return reject(number);
+            resolve(p);
+        });
+    });
+}