Fix arrow highlight 14299

[michaelwalshe]

Description

Update arrow-light.theme to highlight BuiltIn and Extension styles rather than setting as null, applying highlighting more similarly to arrow-dark.theme and other themes. Chosen the same colour as Function style to keep arrow-light fairly minimal.

Fixes #14299

Examples

Example quarto file below which has a few different languages and some builtin & extension style types.

---
highlight-style: arrow
theme:
  light: flatly
  dark: darkly
format:
  html:
    embed-resources: true
---

```typst
#let config = (
  debug: none,
  width: auto,
  visible: true,
)

#let show-box(width: auto, border: none, fill: false) = {
  let has-border = border != none
  box(width: width, stroke: if has-border { border } else { none })
}
```

```python
import re
from statistics import mean

x = abs(-5)

y = dict(
    z = 5,
    b = 1
)

z = mean((1,2,3))

s = re.compile(r"\w{2}\d+[[:upper:]]*")
```

```terraform
locals {
  name    = trimprefix(upper("  hello-world  "), " ")
  offset  = abs(-42)
  items   = sort(["banana", "apple", "cherry"])
  joined  = join(", ", local.items)
  hash    = substr(sha256("my-id"), 0, 8)
}
```

Resulting html under old and new versions of highlighting:

test_old.html

test_new.html

Screenshot samples of differences:

Old:

New: