Support nullish coalescing operator in TypeScript lexer (#1529)

TypeScript has added support for the nullish coalescing operator `??` as
of version 3.7. This commit adds support by prepending a rule to the
`:root` state of lexers that extend the TypeScript common module.
When JavaScript adopts the operator, the rule should be moved into the
`:root` state of the JavaScript lexer and removed from the TypeScript
common module.

lib/rouge/lexers/typescript/common.rb

@@ -33,6 +33,7 @@ def builtins
       def self.extended(base)
         base.prepend :root do
           rule %r/[?][.]/, base::Punctuation
+          rule %r/[?]{2}/, base::Operator
         end
 
         base.prepend :statement do

spec/visual/samples/typescript

@@ -203,3 +203,7 @@ declare module "foo" {
 const x = `Hello world ${a.b}`;
 const y = `Hello world ${a?.b}`;
 const z = `Hello world`;
+
+// Nullish coalescing in template strings
+const content = `Value is ${value ?? 'not set'}`;
+const description = "Highlighting is not broken now";