[508 Accessibility] Support changing the spelling wavy underline color to accommodate color vision impairment

[mtodkari]

• .NET Core Version: (e.g. 3.0 Preview1, or daily build number, use dotnet --info)
• Windows version: (winver) Windows 10
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
• Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No
• Security issues and bugs should be reported privately, learn more via our responsible disclosure guidelines.

Problem description:
The current red underline color for misspelled words is not easily visible for some types of color blindness, so we need to support a way of changing the color. This can be especially bad in dark theme.

MS Office supports customization through a regkey (SpellingWavyUnderlineColor): https://support.microsoft.com/en-us/help/925597/how-to-customize-the-color-of-the-spelling-and-grammar-checker-underli
But it doesn't get respected by WPF.

Looking at the WPF source it looks like this is hard-coded to red:

wpf/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/SpellerHighlightLayer.cs

Line 153 in d49f8dd

Pen pen = new Pen(Brushes.Red, 0.33);

Actual behavior:

Expected behavior:

Minimal repro:

[elyoh]

Duplicate of #1519.