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"
                         },

src/config.rs

@@ -420,6 +420,8 @@ pub struct BookConfig {
     pub description: Option<String>,
     /// Location of the book source relative to the book's root directory.
     pub src: PathBuf,
+    /// Does this book support more than one language?
+    pub multilingual: bool,
     /// The main language of the book.
     pub language: Option<String>,
     /// The direction of text in the book: Left-to-right (LTR) or Right-to-left (RTL).
@@ -434,6 +436,7 @@ impl Default for BookConfig {
             authors: Vec::new(),
             description: None,
             src: PathBuf::from("src"),
+            multilingual: false,
             language: Some(String::from("en")),
             text_direction: None,
         }
@@ -817,6 +820,7 @@ mod tests {
         title = "Some Book"
         authors = ["Michael-F-Bryan <michaelfbryan@gmail.com>"]
         description = "A completely useless book"
+        multilingual = true
         src = "source"
         language = "ja"
 
@@ -855,6 +859,7 @@ mod tests {
             title: Some(String::from("Some Book")),
             authors: vec![String::from("Michael-F-Bryan <michaelfbryan@gmail.com>")],
             description: Some(String::from("A completely useless book")),
+            multilingual: true,
             src: PathBuf::from("source"),
             language: Some(String::from("ja")),
             text_direction: None,

tests/testsuite/init.rs

@@ -28,6 +28,7 @@ All done, no errors...
 [book]
 authors = []
 language = "en"
+multilingual = false
 src = "src"
 
 "#]],
@@ -94,6 +95,7 @@ All done, no errors...
 [book]
 authors = []
 language = "en"
+multilingual = false
 src = "src"
 
 "#]],
@@ -127,6 +129,7 @@ All done, no errors...
 [book]
 authors = []
 language = "en"
+multilingual = false
 src = "src"
 title = "Example title"
 
@@ -179,6 +182,7 @@ fn init_with_custom_book_and_src_locations() {
 [book]
 authors = []
 language = "en"
+multilingual = false
 src = "in"
 
 [build]

tests/testsuite/renderer.rs

@@ -188,6 +188,7 @@ fn backends_receive_render_context_via_stdin() {
     "book": {
       "authors": [],
       "language": "en",
+      "multilingual": false,
       "src": "src"
     },
     "output": {