Invalid default-theme and preferred-dark-theme now cause an error

[hollow-owl]

This causes building to fail if default-theme or preferred-dark-theme is not one of the built-in themes.

If book.toml has an invalid default-theme:

mdbook build outputs an error saying why:

 INFO Book building has started
 INFO Running the html backend
ERROR Failed to deserialize `output.html`
        Caused by: unknown variant `does-not-exist`, expected one of `ayu`, `coal`, `light`, `navy`, `rust`
in `default-theme`

 INFO HTML book written to `/scratch/book`

Fixes #3140

[Chocorean]

The string config for default-themes is now replaced by an enum, and deser of the config now fails on an unknown value.
The list of available themes is now injected into the view rather than hardcoded.
The documentation is updated, and a few tests are added to config.

Seems good to go!

View changes since this review

[GuillaumeGomez]

Small code improvement nit but otherwise looks good to me!

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[hollow-owl]

@rustbot ready

[GuillaumeGomez]

Looks good to me, thanks! One last thing remain to be added: a ui test to check the book building fails. However we don't have such a testsuite currently. Do you want to add it or are you ok with me pushing a commit in your PR adding this testsuite? We actually do have a way to check ui, shame on me for forgetting.

[GuillaumeGomez]

Looks all good to me, thanks!