feat: allow declaring __dirname in config

[sapphi-red]

Description

Currently __dirname/__filename cannot be declared at top level in config files since Vite injects those variables.
The code below is a common pattern but an error will happen.

import path from 'node:path'
import { defineConfig } from 'vite'

const __dirname = path.dirname(new URL(import.meta.url).pathname)

export default defineConfig({})

This PR makes it possible to declare those at top level.
esbuild's define only replaces top-level values (it's not a static replace) so this works.

refs #9147

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• 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.

[bluwy]

😲 The trick is right there all along.