LibWeb: Style computer should never fail

[awesomekling]

We should never fail to compute style for an element.
If a property is somehow invalid, we should not abort style computation.
Basically all use of ErrorOr and TRY in StyleComputer is wrong or at least misused.

As an example, https://twinings.co.uk/ crashes (after a while) right now because some animated CSS transform has an invalid value.

cc @mattco98 @kalenikaliaksandr

[mattco98]

That wasn't clear to me since the cascade functions return ErrorOr. Turns out, it's all because of this one little allocation 😅:

serenity/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

Line 744 in c1dbde7

TRY(custom_properties.try_ensure_capacity(needed_capacity));

#23471 is related; The type of a StyleValue for a property shouldn't be VERIFYd, since it can be anything (probably not for all properties, but it's a good assumption to make regardless).