feat: special case true / false boolean values in variant config

[wolfv]

@pavelzw this should fix #691, although it doesn't attack the problem at the parsing level but later on, when we insert the values into minijinja.

I think I prefer this behavior although it might not be completely obvious.

We could also make it work at parsing time but we would need to change the type of the variant config to allow for either a string or a boolean value. I was too lazy to do that right now :)

We should note that if we change the type later (so that variatn config is a map of bool | string), then hashes would also change, if we don't continue to normalize them to strings.

[pavelzw]

Sounds reasonable to me 👍🏻 I'm looking forward to a CEP for the variant file 😄

[wolfv]

Hmm, this implementation is actually flawed because you can't opt out of the integer / bool thing by using "true" ...

So maybe we should do the proper thing.