3rd party binding generator

[TimLariviere]

Closes #441, #659 and #664

Glossary:

• Assembly = Content of a dll file
• Mapping file = JSON representation of an assembly, feeded to CodeGen through a Generator (e.g. Fabulous.XamarinForms.Generator). Example file: https://github.com/fsprojects/Fabulous/blob/master/Fabulous.XamarinForms/src/Fabulous.XamarinForms/Xamarin.Forms.Core.json
• Bindings = The generated F# code allowing to use the 3rd party Xamarin.Forms controls with Fabulous (DynamicViews)

As explained by #441 (comment), the idea is to write a mapping file with references to other mapping files.
This allows anyone to write their own mapping file to generate bindings for 3rd party libs (like SyncFusion) with the inherited properties/events from Xamarin.Forms.
It's this last part that isn't currently possible.

Writing the following mapping file

{
  "assemblies": [
    "path/to/Xamarin.Forms.Core.dll",
    "path/to/SyncFusion.dll"
  ],

  // This is the new field. It is optional and can contain one or more mapping files
  "baseMappingFiles": [
    "path/to/Xamarin.Forms.Core.json" // Found in the Fabulous.XamarinForms repository
  ],

  "outputNamespace": "Fabulous.XamarinForms.SyncFusion",
  "types": [
    {
      "type": "Syncfusion.Buttons.XForms.SfButton",
      "properties": [
        {
          "source": "IsCheckable"
        },
        {
          "source": "ShowIcon"
        }
      ]
    },
    (...)
  ]
} 

And passing it to Fabulous.XamarinForms.Generator, will result in the matching F# binding code with all the same declared parameters of the inherited button control + the new declared parameters.

View.SfButton(
    text = "SyncFusion button", // Comes from Xamarin.Forms
    showIcon = true // Comes from SyncFusion
)

cc @xperiandri

[TimLariviere]

Currently, it's only a first draft.
I managed to generate this F# binding code from this mapping file.
The TestButton control is declared here

It's only missing the correct open statements to be compilable. But should be working once added manually.
open statements are automatically generated now.

[TimLariviere]

Managed to successfully move OxyPlot, SkiaSharp and VideoManager under automatically generated bindings

[TimLariviere]

Implemented #659 and #664 for FFImageLoading

[TimLariviere]

This PR should be ok now.

@SergejDK Could you please review it and let me know what you think?
Thanks.

[SergejDK]

All in all it looks very good and it seems to work for so much as I could test it :)
Thanks for this feature!

[SergejDK]

@TimLariviere
About #664 should we check the ViewUpdaters.fs, too? I mean we use the defaultValue there and maybe we should use ClearProperty in this file. What dou you think?

[TimLariviere]

About #664 should we check the ViewUpdaters.fs, too? I mean we use the defaultValue there and maybe we should use ClearProperty in this file. What dou you think?

Yes! You're completely right. Forgot to check this file.
Will do.