Reformat compiler error/warning list

Hi,

I would like to suggest that the list on the left panel is reformatted to be displayed as a tree/grouped list with the following groups:

- Compiler errors
- Compiler warnings
- Level 1
- Level 2
- Level 3
- Level 4
- Level 5

and then to have directly under it the compiler issue ID. This would give a unifed experience with analyzer documentation (i.e. https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/).

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#### Document Details

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* ID: fc586192-534e-0108-d265-c4f2f6947c57
* Version Independent ID: 4d39f536-5d78-335b-3525-d4b581c28fdc
* Content: [C# Compiler Errors](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-messages/)
* Content Source: [docs/csharp/language-reference/compiler-messages/index.md](https://github.com/dotnet/docs/blob/master/docs/csharp/language-reference/compiler-messages/index.md)
* Product: **dotnet-csharp**
* Technology: **csharp-diagnostics**
* GitHub Login: @BillWagner
* Microsoft Alias: **wiwagn**