Add support for SVG favicons

[kiddailey]

This pull request addresses issue #15 adding support for SVG icon types that are found by a scan. This is a pretty big change and hopefully is a good solution. I'm open to suggestions, changes or improvements of course!

A few key things to note:

• Based on the main branch It is incompatible with the Async branch pull requests. Some work was required to port these changes, and it is available (see note below)
• 100% backwards compatible No code changes in the calling app required.
• IconImage abstraction Since their data and handling are unique, BitmapIconImage and SVGIconImage are new classes that implement the new IIconImage interface.
• New IconSize.Scaleable When scanning, indicates that the icon is scaleable and can always match PerfectSize.
• SkiaSharp.svg package SVG support is fully supported in SkiaSharp, however, it requires this (tiny) official extension package
• DEBUG conditional Testing this with real sites can be annoying as some block requests and you are left with just favicon.ico in the scan results. I've added a DEBUG conditional compile to set a "full" UserAgent that hopefully works around most of these instances. This means that in release mode, the caller will need to set the UserAgent themselves. (See issue #27 for more info)

There are many sites that include SVG icons, but here are a few to test with:

• twitter.com/x.com
• github.com
• css-tricks.com
• medium.com

NOTE: I have a consolidated branch containing the SVG, Async and HttpClient pull requests into one if preferred. Since each PR is based off of the mainline, it does take a little work to combine them. Let me know

[kiddailey]

Noticed that some SVG favicons were being "clipped" and it appears it is due to ViewBoxes that don't match the canvas size. I've added a change to ignore the ViewBox and strictly use the canvas size.

Some vector editing tools may be setting the ViewBox when saving the SVG to remember the last view in the editor. I also don't see the same clipping in browsers when accessing the SVGs directly, or in the favicon, so ignoring it seems to be proper.