[boom] isBoom is a type-guard (#22918)

Instead of just returning boolean, the `isBoom` function can and should be a type-guard, which enables the obvious benefits.

types/boom/boom-tests.ts

@@ -144,6 +144,12 @@ const isBoomError = new Boom.Boom('test')
 
 Boom.isBoom(isBoomError);
 
+const maybeBoom = <any>new Boom.Boom('test');
+if(Boom.isBoom(maybeBoom)) {
+    // isBoom is a type guard that allows accessing these properties:
+    maybeBoom.output.headers;
+}
+
 // constructor
 
 const constructorError: Boom.Boom = new Boom.Boom('test');

types/boom/index.d.ts

@@ -86,7 +86,7 @@ declare namespace Boom {
      * Identifies whether an error is a Boom object. Same as calling instanceof Boom.
      * @param error the error object to identify.
      */
-    export function isBoom(error: Error): boolean
+    export function isBoom(error: Error): error is Boom
 
     // 4xx
     /**