Builds fail when ÷ character is formatted as code without syntax highlighting

[tobyhodges]

(Derived from a report by @knarrff on Slack)

Something catastrophic happens to the lesson build when a ÷ character is included in a page, formatted as code but with no language specified for syntax highlighting.

I created a minimal example at https://github.com/tobyhodges/refactored-sniffle, demonstrating how

```julia
a ÷ b
```

works fine, but

```
a ÷ b
```

causes something bad to happen to the build.
(Not shown, but reproduced on my local system, is that inline code (i.e. enclosed in single backticks) fails without a language class applied, but works fine with e.g. {.julia} added afterwards.)

The GitHub Actions logs on that example repo suggest that the failure happens even before the lesson site starts to be built. I cannot confirm that, but have been able to reproduce the error with a local build of the linked minimal example.

[tobyhodges]

Following up, this seems to be a problem with pkgdown: a second minimal example using pkgdown directly gives the same problem when running pkgdown::build_site().

[zkamvar]

Linking r-lib/downlit#189 so that it's clear where this is being fixed.

[tobyhodges]

The new version of downlit has been released and have confirmed that lesson builds are now succeeding.