Use more lightweight cc-mode scanning function for js2-mode

`scan-identifiers` isn't the best scan-fn (see also ankurdave#94) as it moves not
just through changed properties. Let's use the cc-mode one, which goes
exclusively through face changes while searching for declared
identifiers.

Fixes: ankurdave#40

color-identifiers-mode.el

@@ -236,6 +236,8 @@ For cc-mode support within color-identifiers-mode."
               "\\_<\\([a-zA-Z_$]\\(?:\\s_\\|\\sw\\)*\\)"
               (nil font-lock-variable-name-face))))
 
+(color-identifiers:set-declaration-scan-fn
+ 'js2-mode 'color-identifiers:cc-mode-get-declarations)
 (add-to-list
  'color-identifiers:modes-alist
  `(js2-mode . (,color-identifiers:re-not-inside-class-access