chore(bench): Setup benchmark script for testing browser open timing (#917)

scripts/benchmarks/browser-startup.patch

@@ -0,0 +1,26 @@
+diff --git a/packages/wxt/src/core/runners/web-ext.ts b/packages/wxt/src/core/runners/web-ext.ts
+index 09819c3..a0f0df2 100644
+--- a/packages/wxt/src/core/runners/web-ext.ts
++++ b/packages/wxt/src/core/runners/web-ext.ts
+@@ -3,6 +3,8 @@ import { ExtensionRunner } from '../../types';
+ import { formatDuration } from '../utils/time';
+ import defu from 'defu';
+ import { wxt } from '../wxt';
++import fs from 'node:fs';
++import stream from 'node:stream/promises';
+ 
+ /**
+  * Create an `ExtensionRunner` backed by `web-ext`.
+@@ -78,6 +80,12 @@ export function createWebExtRunner(): ExtensionRunner {
+ 
+       const duration = Date.now() - startTime;
+       wxt.logger.success(`Opened browser in ${formatDuration(duration)}`);
++      await runner.exit();
++      const s = fs.createWriteStream('stats.txt', { flags: 'a' });
++      s.write(duration + ' ');
++      s.end();
++      await stream.finished(s);
++      process.exit(0);
+     },
+ 
+     async closeBrowser() {

scripts/benchmarks/browser-startup.sh

@@ -0,0 +1,41 @@
+#!/bin/bash
+set -e
+
+#
+# Benchmark how long it takes for the browser to open in dev mode.
+#
+# To run:
+#   cd <root>
+#   ./scripts/benchmarks/browser-startup.sh
+#
+# You can set N below to change the number of samples per git ref.
+#
+
+N=20
+
+function benchmark_ref() {
+    # Prep
+    git checkout $1
+    pnpm buildc clean
+    git apply scripts/benchmarks/browser-startup.patch
+    pnpm i --ignore-scripts
+    pnpm -r --filter wxt build
+    echo -n "$1 " >> stats.txt
+
+    # Run benchmark
+    for i in $(seq $N); do
+        pnpm wxt packages/wxt-demo
+    done
+    git checkout HEAD -- packages/wxt/src/core/runners/web-ext.ts pnpm-lock.yaml
+    echo "" >> stats.txt
+}
+
+rm -f stats.txt
+
+benchmark_ref "HEAD"
+
+# Benchmark a commit:
+#   benchmark_ref "3109bba"
+#
+# Benchmark a version:
+#   benchmark_ref "v0.19.0"