Revert: Remove the book.multilingual field

This reverts https://github.com/rust-lang/mdBook/pull/2646/ because I
overlooked that this is a public field in a public struct, which would
be a breaking API change.

examples/nop-preprocessor.rs

@@ -117,6 +117,7 @@ mod nop_lib {
                         "book": {
                             "authors": ["AUTHOR"],
                             "language": "en",
+                            "multilingual": false,
                             "src": "src",
                             "title": "TITLE"
                         },