Add an entry on the coding style for primary constructors

[jkoritzinsky]

Based on offline discussions, add an entry about primary constructors.

[Copilot]

Pull Request Overview

Adds a new coding guideline entry for naming and using primary constructor parameters.

• Introduces guideline #20 on camelCase naming for primary constructor parameters and when to assign them to fields.
• Provides examples demonstrating correct parameter names and field assignments.

Comments suppressed due to low confidence (1)

docs/coding-guidelines/coding-style.md:36

• The example assigns _items = item, but the parameter is named items, so item is undefined. It should read _items = items;.

private readonly IEnumerable<T> _items = item;

[stephentoub]

/ba-g markdown-only change