New issue template for bugs related to Nullable Reference Types

[github-actions[bot]]

:white_check_mark: No release notes required

[vzarytovskii]

We can also use yml template, it will render as form:

name: Nullness-related report
description: File a bug report.
title: "[Nullness]: "
labels: ["bug", "Needs-Triage", "Area-Nullness"]
body:
  - type: markdown
    attributes:
      value: |
        Please check at [Nullable Reference Types RFC](https://github.com/fsharp/fslang-design/blob/main/RFCs/FS-1060-nullable-reference-types.md) if this issue isn't a known limitation, such as missing flow-control analysis after branching constructs like `if` or `while`.
  - type: input
    id: description
    attributes:
      label: Description
      description: Please provide a succinct description of the issue
      placeholder: Description
    validations:
      required: true
  - type: checkboxes
    id: categories
    attributes:
      label: Choose one or more from the following categories of impact
      options:
        - label: Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
        - label: Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
        - label: Breaking change related to older `null` constructs in code not using the checknulls switch.
        - label: Breaking change related to generic code and explicit type constraints (`null`, `not null`).
        - label: Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotatins).
        - label: C#/F# interop issue related to nullness metadata.
        - label: Other (none of the categories above apply).
    validations:
      required: true
  - type: dropdown
    id: os
    attributes:
      label: Operating System
      description: What operating system are you running?
      options:
        - Windows (Default)
        - Linux
        - macOS
      default: 0
    validations:
      required: true
  - type: dropdown
    id: dotnet-kind
    attributes:
      label: What .NET runtime/SDK kind are you seeing the issue on
      multiple: false
      options:
        - .NET SDK (.NET Core, .NET 5+)
        - .NET Framework
      default: 0
    validations:
      required: true
  - type: input
    id: dotnet-version
    attributes:
      label: .NET Runtime/SDK version
      placeholder: e.g. .NET 9 or .NET SDK 9.0.100