fix: Restore CLI port in use prompt feature (#25863)

* fix: Restore CLI port in use prompt feature

Seems like it got accidentally removed during a big PR?

* fix: Ensure port var types are the same

The CLI option and default value for `port` is a string, despite the TypeScript typing the arg to `number`.

Unclear if `port` should be a `number` elsewhere, so checking for and converting to a number within the utility.

packages/gatsby/src/commands/develop.ts

@@ -5,6 +5,7 @@ import tmp from "tmp"
 import { spawn, ChildProcess } from "child_process"
 import chokidar from "chokidar"
 import getRandomPort from "detect-port"
+import { detectPortInUseAndPrompt } from "../utils/detect-port-in-use-and-prompt"
 import socket from "socket.io"
 import fs from "fs-extra"
 import { isCI, slash } from "gatsby-core-utils"
@@ -163,6 +164,17 @@ const REGEX_IP = /^(?:(?:25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])\.){3}(?:25
 
 module.exports = async (program: IProgram): Promise<void> => {
   const developProcessPath = slash(require.resolve(`./develop-process`))
+
+  try {
+    program.port = await detectPortInUseAndPrompt(program.port)
+  } catch (e) {
+    if (e.message === `USER_REJECTED`) {
+      process.exit(0)
+    }
+
+    throw e
+  }
+
   // Run the actual develop server on a random port, and the proxy on the program port
   // which users will access
   const proxyPort = program.port

packages/gatsby/src/utils/detect-port-in-use-and-prompt.ts

@@ -5,11 +5,11 @@ import prompts from "prompts"
 export const detectPortInUseAndPrompt = async (
   port: number
 ): Promise<number> => {
-  let foundPort = port
-  const detectedPort = await detectPort(port).catch((err: Error) =>
+  let foundPort = typeof port === `string` ? parseInt(port, 10) : port
+  const detectedPort = await detectPort(foundPort).catch((err: Error) =>
     report.panic(err)
   )
-  if (port !== detectedPort) {
+  if (foundPort !== detectedPort) {
     report.log(`\nSomething is already running at port ${port}`)
     const response = await prompts({
       type: `confirm`,