Refactor settings arrays to ISet<T> (to ensure unique values and make them easily mutable)

[ronaldbarendse]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Most configuration/options classes currently bind certain settings to arrays, which is mostly fine when configuring them from the appsettings.json files, but make adding new or removing default items in code a lot harder.

Consider removing TIFF and adding PBM and TGA image file types on the ContentImagingSettings configuration. Even when using the spread element .. to make adding items to the array easier, removing an existing item requires filtering the existing array:

using Umbraco.Cms.Core.Composing;
using Umbraco.Cms.Core.Configuration.Models;

public class ImageFileTypesComposer : IComposer
{
    public void Compose(IUmbracoBuilder builder)
        => builder.Services.Configure<ContentImagingSettings>(options =>
        {
            options.ImageFileTypes =
            [
                // Remove TIFF
                ..options.ImageFileTypes.Where(x => x == "tiff"),
                // Add PBM and TGA
                "pbm",
                "tga"
            ];
        });
}

With this PR applied, the ImageFileTypes is now an ISet<string>, which can easily be mutated:

using Umbraco.Cms.Core.Composing;
using Umbraco.Cms.Core.Configuration.Models;

public class ImageFileTypesComposer : IComposer
{
    public void Compose(IUmbracoBuilder builder)
        => builder.Services.Configure<ContentImagingSettings>(options =>
        {
            // Remove TIFF
            options.ImageFileTypes.Remove("tiff");

            // Add PBM and TGA
            options.ImageFileTypes.Add("pbm");
            options.ImageFileTypes.Add("tga");
        });
}

As a bonus, this won't add duplicate items to the set 🙌🏻 Additional improvements can be made by refactoring some settings to dictionaries, so they can be configured by unique key (e.g. RTE commands by alias, which would fix issue #14405). Other settings might benefit from being refactored as well, like the reserved paths and URLs (which would fix issue #12965).

[bergmania]

I guess we should announce this change, and wait with it for v16, even that it is super nice. It is a breaking change, with no much notice.

[bergmania]

I created the following announcement
umbraco/Announcements#21