fix(plugin-legacy): respect entryFileNames for polyfill chunks

[sapphi-red]

Potentially breaking change: vite/legacy-polyfills in manifest is now vite/legacy-polyfills-legacy. polyfills-modern.hash.js is now polyfills.hash.js.

Description

legacy and modern polyfill chunk file names now respect entryFileNames.
As a byproduct, conflicted entry in manifest was solved. I will describe this in the additional context.

I tested with pnpm build:custom-filename in playground/legacy.

close #4221
refs #8180

Additional context

When modernPolyfill is enabled, vite/legacy-polyfills in manifest.json points to modern polyfill chunk. Also there was no entry pointing to legacy polyfill chunk.
When it is disabled, it points to legacy polyfill chunk.

It was like this.

  "vite/legacy-polyfills": {
    "file": "assets/polyfills-modern.792b0ede.js",
    "src": "vite/legacy-polyfills",
    "isEntry": true
  },

This PR changes it to

  "vite/legacy-polyfills-legacy": {
    "file": "assets/polyfills-legacy.378fc635.js",
    "src": "vite/legacy-polyfills-legacy",
    "isEntry": true
  },
  "vite/legacy-polyfills": {
    "file": "assets/polyfills.4dce788d.js",
    "src": "vite/legacy-polyfills",
    "isEntry": true
  }

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

(maybe it's a feat than a fix?)

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.