Introduce Unity page for Inspection Severity

Currently, Unity settings for Inspection Severity are shown in the C# settings page, grouped under "Unity", "Unity Burst Compiler Warnings" and "Unity Performance Inspections". Most of these settings relate to C# code, but at least one is for `.asmdef` files ("File name should match the name of the assembly").

Create a "Unity" settings page for Unity specific inspections, similar to "ASP.NET" (all others are languages, e.g. "C#", "Razor", "XAML", etc.). The Unity page can have more granular groups, such as "Best practices", "Potential Code Quality Issues", "Performance", "Assembly Definition Files", etc.

Compare the existing C# settings page and the ASP.NET settings page to understand how inspection pages and groups are organized. Identify the current Unity, Unity Burst Compiler Warnings, and Unity Performance Inspections groups, then define a dedicated Unity page with appropriate groups, including Assembly Definition Files; done means Unity-specific inspections no longer appear under the C# page.

Written by the indexing model from the issue text.