better concurrency debugging, --concurrency flag for npx quartz build

quartz/worker.ts

@@ -1,7 +1,10 @@
+import sourceMapSupport from "source-map-support"
+sourceMapSupport.install(options)
 import cfg from "../quartz.config"
 import { Argv, BuildCtx } from "./ctx"
 import { FilePath, ServerSlug } from "./path"
 import { createFileParser, createProcessor } from "./processors/parse"
+import { options } from "./sourcemap"
 
 // only called from worker thread
 export async function parseFiles(argv: Argv, fps: FilePath[], allSlugs: ServerSlug[]) {