[Editor] Cheat the stamp editor icon to make a linter in m-c happy

[calixteman]

At some point we won't use this icon anymore in the build-in pdf viewer but we'll the one already in the m-c tree.

[Snuffleupagus]

At some point we won't use this icon anymore in the build-in pdf viewer but we'll the one already in the m-c tree.

Note that this ought to be fairly simple to implement actually, since you'd only need to do something along these lines:

• Replace the existing CSS variable with

  /*#if MOZCENTRAL*/
  --toolbarButton-editorStamp-icon: url(appropriate mozilla-central path here...);
  /*#else*/
  --toolbarButton-editorStamp-icon: url(images/toolbarButton-editorStamp.svg);
  /*#endif*/

• Update the list of resources to include in the build

  const MOZCENTRAL_WEB_FILES = [
    ...COMMON_WEB_FILES,
    "!web/images/toolbarButton-editorStamp.svg",
    "!web/images/toolbarButton-openFile.svg",
  ];

[calixteman]

At some point we won't use this icon anymore in the build-in pdf viewer but we'll the one already in the m-c tree.

Note that this ought to be fairly simple to implement actually, since you'd only need to do something along these lines:

* Replace [the existing CSS variable](https://github.com/mozilla/pdf.js/blob/35202ec0f390e63035aba6906f454fd395650e58/web/viewer.css#L83) with
  ```css
  /*#if MOZCENTRAL*/
  --toolbarButton-editorStamp-icon: url(appropriate mozilla-central path here...);
  /*#else*/
  --toolbarButton-editorStamp-icon: url(images/toolbarButton-editorStamp.svg);
  /*#endif*/
  ```

* Update [the list of resources](https://github.com/mozilla/pdf.js/blob/35202ec0f390e63035aba6906f454fd395650e58/gulpfile.js#L1318-L1321) to include in the build
  ```js
  const MOZCENTRAL_WEB_FILES = [
    ...COMMON_WEB_FILES,
    "!web/images/toolbarButton-editorStamp.svg",
    "!web/images/toolbarButton-openFile.svg",
  ];
  ```

Yeah, I tried --toolbarButton-editorStamp-icon: url(chrome://browser/skin/canvas.svg); but it wasn't working.
That said there are few other icons to de-dup, I filed this bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1842746