rust: Add support for string continuation escape

Closes #2102

Author: Jisu-Woniu

lib/rouge/lexers/rust.rb

@@ -253,6 +253,7 @@ def macro_closed?
       state :string do
         rule %r/"/, Str, :pop!
         rule escapes, Str::Escape
+        rule %r/\\\n[ \t\r\n]*/, Str::Escape
         rule %r/[^"\\]+/m, Str
       end
     end

spec/visual/samples/rust

@@ -38,6 +38,9 @@ println!(r###"r##"r#""#"##"###);
 println!(r#"
   "New line in a raw string"
 "#);
+println!("String continuation\
+
+    escape");
 
 trait Iterable<A> {
     fn iterate(&self, blk: impl FnMut(&A) -> bool);