The legacy attrset option type for `firefox.profiles.<name>.bookmarks`
was accidentally removed in 9d55428. This adds back support for this
type by refactoring the bookmarks submodule. This also adds a new test
ensuring this won't happen again.
This splits the bookmarks submodule into a seperate file, to make it easier to maintain (like how the search module was previously split out in #5697).
This also refactors bookmarks to require a new force option, to be more explicit about overriding existing bookmarks.
