Custom Visual Bell

# Allowing User Customization of the Visual Bell
I came across [this thread](https://github.com/microsoft/terminal/issues/8366) in which custom bell sounds were added, and realized that it could also be useful to customize the visual bell as well. I think the most straightforward customizations would be:

1) Allow changing the color, intensity, and duration of the default behavior (for instance, a normally black terminal flashing red briefly instead of grey) and/or
2) Adding, or overriding any background image with one indicating failure (see below)

Start with this issue and the referenced issue #8366 about custom bell sounds; no implementation files, tests, or entry points are identified in the report. First clarify which visual-bell behavior is intended, then define completion as an agreed, testable scope for color, intensity, duration, and/or a failure background image.

Written by the indexing model from the issue text.