Outline-Iterator: add style and color information to OutlineItem

[HeimMatthias]

PDF Outline Items can have an RGB color encoding and be bold and/or italicized. This information cannot be get / set via the OutlineItem interface and should be added as one of the properties of this object, so that it can be read via document.loadOutline(); and outlineIterator.item();. and written via outlineIterator.insert(item); and outlineIterator.update(item);.

I propose to keep the new interface properties as close to the Adobe implementation as possible, so
.color (https://opensource.adobe.com/dc-acrobat-sdk-docs/library/jsapiref/JS_API_AcroJS.html#color), would be a color array with the color space before it. But I understand that MuPDF handles colors differently, and the implementation might have to mirror that behaviour (https://mupdfjs.readthedocs.io/en/latest/glossary/index.html#colors) (though accessing outline items is usually device-independent, so I'm not sure how that would work in practice). For all intents and purposes an RGB-implementation would suffice, since the PDF-standard (PDF32000_2008, p. 369) only allows DeviceRGB anyway, but the length of the color-array itself could be used to define the color space.
.style, where "0 is normal, 1 is italic, 2 is bold, and 3 is bold-italic" (https://opensource.adobe.com/dc-acrobat-sdk-docs/library/jsapiref/JS_API_AcroJS.html#id247)

I understand that this information can already be retrieved or set via Object access, using the C and F entries of the corresponding pdf object, but this is more complex than it should be. I have implemented my own solution in this gist: https://gist.github.com/HeimMatthias/f628b8dd2ed9062e72cea544fffb3f40
Because my script's creates an Outline-Iterator and traverses the entire outline-tree every time to access the outline-item's corresponding object, however, it tends to fail with many complex documents. But this method works fine without the abstraction (i.e. if I follow the outline-objects from the trailer while parsing the outline).

[jamie-lemon]

@HeimMatthias Thanks for this - this looks really interesting! I'm going to test the functionality in the gist you supplied and get back to you. If all good I think we could then hopefully integrate this formally via a PR. :)