docs: add warning on Worker import with constructor limitations

[malta895]

Description

The explanation on how to import Workers with Constructor is not explaining how Vite actually parses the user code at build time.

I tried to import a SharedWorker like this:

const WORKER_NAME = 'MyWorkerName'
const workerUrl = new URL('./worker.js', import.meta.url)
const worker = new SharedWorker(workerUrl, {
    type: 'module',
    name: WORKER_NAME
))

This isn't working for two reasons:

• the name option isn't a static value, and it triggers a build error
• workerUrl is declared in a separated variable, and this is not recognized by Vite

The latter does not even trigger an error, but just a warning. This ends up in a potentially broken build, without the Worker being bundled correctly.

Reference to the relevant code: https://github.com/vitejs/vite/blob/56ae92c33cba6c86ab5819877c19b9ea39f7121b/packages/vite/src/node/plugins/workerImportMetaUrl.ts

I added a warning that explains this behavior.

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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

• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• 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).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

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