[3.1.0] `this` inside plugin hooks no loger points to the plugin object

[csr632]

Describe the bug

In this PR that just published: https://github.com/vitejs/vite/pull/9634/files#r965058188
If plugin authors use this in their plugin hooks to refer to the plugin object, like this:
https://github.com/brillout/vite-plugin-mdx/blob/ee4a732f19bd8ee072dc5257b96c3012f2cd6d75/src/index.ts#L52

After upgrading to vite@3.1.0, we get error when running vite serve:

TypeError: Cannot set properties of undefined (setting 'transform')
    at configResolved (/.../vite-plugin-mdx/dist/index.js:47:28)

stackblitz demo: https://stackblitz.com/edit/vitejs-vite-8xpzra?file=package.json&terminal=dev

Previous version works: vite@3.0.9.

Reproduction

https://stackblitz.com/edit/vitejs-vite-8xpzra?file=package.json&terminal=dev

System Info

vite@3.1.0

Used Package Manager

npm

Logs

No response

Validations

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• The provided reproduction is a minimal reproducible example of the bug.

[antfu]

This is expected, plugins should not rely on hook's this to access the plugin object. #9885