vfs: restore sea.js to match upstream main

sea.js was treating isSea as a boolean property, but the C++ binding
was restored to expose isSea as a function. This mismatch caused
getAssetKeys() to crash with a C++ assertion failure when called
outside a SEA, because the JS guard `if (!isSeaFlag)` never triggered
(a function reference is always truthy).

Restore sea.js from upstream/main to use isSea() as a function call.

Author: mcollina

lib/sea.js

@@ -3,7 +3,7 @@ const {
   ArrayBufferPrototypeSlice,
 } = primordials;
 
-const { isSea: isSeaFlag, getAsset: getAssetInternal, getAssetKeys: getAssetKeysInternal } = internalBinding('sea');
+const { isSea, getAsset: getAssetInternal, getAssetKeys: getAssetKeysInternal } = internalBinding('sea');
 const { TextDecoder } = require('internal/encoding');
 const { validateString } = require('internal/validators');
 const {
@@ -23,7 +23,7 @@ const { Blob } = require('internal/blob');
 function getRawAsset(key) {
   validateString(key, 'key');
 
-  if (!isSeaFlag) {
+  if (!isSea()) {
     throw new ERR_NOT_IN_SINGLE_EXECUTABLE_APPLICATION();
   }
 
@@ -74,17 +74,13 @@ function getAssetAsBlob(key, options) {
  * @returns {string[]}
  */
 function getAssetKeys() {
-  if (!isSeaFlag) {
+  if (!isSea()) {
     throw new ERR_NOT_IN_SINGLE_EXECUTABLE_APPLICATION();
   }
 
   return getAssetKeysInternal() || [];
 }
 
-function isSea() {
-  return isSeaFlag;
-}
-
 module.exports = {
   isSea,
   getAsset,