Add support for optional chaining operator to TypeScript lexer (#1475)

TypeScript has added support for the optional chaining operator `?.` as
of version 3.7. This commit adds support by prepending a rule to the
`:root` state of the TypeScript lexer. Eventually, when JavaScript
adopts the operator, the rule should be moved into the `:root` state
there and removed from this lexer.

lib/rouge/lexers/typescript.rb

@@ -19,6 +19,10 @@ class Typescript < Javascript
 
       mimetypes 'text/typescript'
 
+      prepend :root do
+        rule %r/[?][.]/, Punctuation
+      end
+
       prepend :statement do
         rule %r/(#{Javascript.id_regex})(\??)(\s*)(:)/ do
           groups Name::Label, Punctuation, Text, Punctuation

spec/visual/samples/typescript

@@ -198,3 +198,8 @@ declare module "foo" {
     [K in keyof IFoo]?: IFoo[K]
   }
 }
+
+// Template strings
+const x = `Hello world ${a.b}`;
+const y = `Hello world ${a?.b}`;
+const z = `Hello world`;