feat: add background-color to <code> blocks

Right now neochat doesn't seem to do any significant distinction for backticks surrounded words or triple backtics code blocks. When looking at the source in a neochat message I saw neochat uses <code> blocks for those situations.

With this change now we have a different background for those expressions.

Light:

Dark:

I don't consider it perfect but I think it's better than what we currently have.

Note: I haven't tested with bubbles/no bubbles. It would be great if somebody else could test and check how it works there.