fix(css): include .css?url in assets field of manifest

[sapphi-red]

Description

fixes #17484

The manifest in the repro changes from

  "_stuff.js": {
    "file": "chunks/stuff.js",
    "name": "stuff",
    "assets": [
      "assets/text.txt"
    ]
  }

to

  "_stuff.js": {
    "file": "chunks/stuff.js",
    "name": "stuff",
    "assets": [
      "assets/text.txt",
      "assets/styles.css"
    ]
  },