no styles applied to Preact app

## Environment

node.js v16.10.0
npm 7.24.0
@linaria/core 3.0.0-beta.15
@linaria/react 3.0.0-beta.15
preact-cli 3.3.5

## Description

following the documentation at https://github.com/callstack/linaria/blob/master/docs/CONFIGURATION.md#preact , I ran into a couple of problems.

- problem no.1 was a unresolved dependency @linaria/shaker .

this appeared when attempting to start a dev server with `npm run dev`. it was easily solved via `npm install`.

- problem no.2 was that styles created by linaria do not appear on my components, though the styles created by CSS Module files do appear.

styled elements have classnames like `.c1qu2euk` but stylesheets contain selectors like `.c1qu2euk__Oj3Ly` (i.e. something which seems related but is definitely different). this happens in both the dev server launched via `npm run dev` and in static assets created via `npm run build`.

I have been unable to solve the second problem. I tried setting `classNameSlug` in linaria.config.js , which did indeed change the slug but did not change the mismatch between class names and CSS selectors.

## Reproducible Demo

1. clone the repo at https://github.com/orzechowskid/linaria-preact.git
2. `npm install`
3. `npm run dev`
4. browse to http://localhost:8080

the homepage (src/routes/home/index.js) should render a `