Highlighting lines

Curly braces after the language mark lines:

```js {2}
const a = 1
const b = 2
const c = 3
```

const a = 1
const b = 2
const c = 3

Several lines and ranges

```js {1,3-4}
const a = 1
const b = 2
const c = 3
const d = 4
```

const a = 1
const b = 2
const c = 3
const d = 4

In this template

The highlighted line gets a tinted surface and a bar on the left edge. A tint alone is too weak on a code block that is tinted anyway.