watch: watch env files in --watch mode

observe env files in --watch mode through the watcher's filterFile and
forward changes to the childProcess

Fixes: nodejs#54001

lib/internal/main/watch_mode.js

@@ -7,6 +7,7 @@ const {
   ArrayPrototypePushApply,
   ArrayPrototypeSlice,
   StringPrototypeStartsWith,
+  StringPrototypeSplit,
 } = primordials;
 
 const {
@@ -22,10 +23,11 @@ const { FilesWatcher } = require('internal/watch_mode/files_watcher');
 const { green, blue, red, white, clear } = require('internal/util/colors');
 
 const { spawn } = require('child_process');
-const { inspect } = require('util');
+const { inspect, parseEnv } = require('util');
 const { setTimeout, clearTimeout } = require('timers');
 const { resolve } = require('path');
 const { once } = require('events');
+const { readFileSync } = require('fs');
 
 prepareMainThreadExecution(false, false);
 markBootstrapComplete();
@@ -64,11 +66,25 @@ let exited;
 
 function start() {
   exited = false;
+
+  let kEnv = {};
+  for (let i = 0; i < process.execArgv.length; i++) {
+    const arg = process.execArgv[i];
+    if (StringPrototypeStartsWith(arg, '--env-file')) {
+      const [,envFileName] = StringPrototypeSplit(arg, '=');
+      const envFile = resolve(envFileName);
+      watcher.filterFile(envFile);
+      const envContent = parseEnv(readFileSync(envFile, 'utf8'));
+      kEnv = {...kEnv, ...envContent}
+    }
+  }
+
   const stdio = kShouldFilterModules ? ['inherit', 'inherit', 'inherit', 'ipc'] : 'inherit';
   child = spawn(process.execPath, argsWithoutWatchOptions, {
     stdio,
     env: {
       ...process.env,
+      ...kEnv,
       WATCH_REPORT_DEPENDENCIES: '1',
     },
   });