Update dev dependencies

## Goal

Update all outdated devDependencies to their latest major versions and perform the necessary configuration migrations to keep the project maintainable and secure.

## Problem

Most devDependencies are multiple major versions behind and some introduce security vulnerabilities:

| Package | Current | Latest |
|---|---|---|
| `eslint` | `^6.8.0` | `^10.0.3` |
| `husky` | `^4.2.3` | `^9.1.7` |
| `jasmine-node` | `^1.16.0` | `^3.0.0` (CVEs, see below) |
| `lint-staged` | `^10.0.8` | `^16.3.2` |
| `postcss` | `^8.0.0` | `^8.5.8` |
| `prettier` | `^1.19.1` | `^3.8.1` |

Additionally, the CI workflow tests against Node.js 8, 10, and 12 — all of which are End-of-Life.

## Plan

### Replace `jasmine-node` with `jasmine`

`jasmine-node@3` (the latest version) has unresolvable critical CVEs in its transitive dependencies (`underscore`, `minimatch`). Migrate to the official `jasmine` package, which is the actively maintained successor and a drop-in replacement for this test suite.

### Migrate ESLint to flat config

ESLint v9+ dropped support for the legacy `.eslintrc.*` format:

- Delete `.eslintrc.js` and `example/.eslintrc.js`
- Create `eslint.config.js` with equivalent rules

### Migrate Husky to v9

Husky v5+ dropped the `package.json`-based hooks format:

- Remove `husky.hooks` block from `package.json`
- Add `"prepare": "husky"` script to `package.json`
- Create `.husky/pre-commit` shell script

### Update CI workflow

- Update Node.js matrix from `[8.x, 10.x, 12.x]` to `[18.x, 20.x, 22.x]`
- Bump `actions/checkout@v2` → `v4` and `actions/setup-node@v1` → `v4`
- Replace `npm install` with `npm ci` for reproducible installs
- Add `cache: 'npm'` to speed up CI runs

### Fix stale test assertion

The test *"should remain unitless if 0"* was never updated after commit `a90b209` changed how zero values are handled. Correct the expected value (`0px` → `0rem`).