Consider making IDE0044 (Make field readonly) precondition more strict/accurate

**Brief description:**

Currently, IDE0044 is emitted whenever there's no assignment to a field outside of the constructor. However, adding `readonly` can alter semantics as methods might mutate a `struct`.

**Languages applicable:** Applicable to C# and VB.

**Code example that the analyzer should report:**

```csharp
public interface IFoo { void Init(); void DoWork(); }

public class C where T : struct, IFoo
{
// IDE0044: Make field readonly
// Prior to this issue: IDE0044 is reported.
// Suggestion: Do not report IDE0044 here.
private T foo;

// Two initialization modes available.
public C() { foo.Init(); }
public C(T f) { foo = f; }

public void DoTwice()
{
foo.DoWork();
foo.DoWork();
}
}
```

A real-world scenario is when `T` above represents a hash function that's resampleable, and `Init` samples one from the family.

**Additional information:**

The condition of reporting IDE0044 roughly has some similarities with that of simplifying identity conversions. (See #85201 ) I propose the following condition to be used for non-byref fields.

- If the field is assigned to outside of the constructors, **do not report** IDE0044 (this rule already exists). Otherwise, consider the rules below until a decision is first made.
- If the field is of reference type (a type-parameter type constrained to `class` or `class?` or a base class that's not `Enum`, or a reference type constructed from 0 or more type parameters), then IDE0044 **should be reported**.
- If the field is of value type that is known to be `readonly` (in this case, it must be a value type constructed from 0 or more type parameters that is not itself a type parameter), then IDE0044 **should be reported**.
- If the field type is pointer, then IDE0044 **should be reported**.
- Otherwise, **do not report** IDE0044.

Moreover, currently IDE0044 is not reported for byref fields. The proposed rule for byref fields is as follows.

- If the field is ref-assigned-to outside of the constructors, **do not report** IDE0044.
- Otherwise, IDE0044 **should be reported**.

**Documentation requirements:**

When this analyzer is implemented, it must be documented by following the steps at [Documentation for IDE CodeStyle analyzers](https://github.com/dotnet/roslyn/blob/main/docs/contributing/Documentation%20for%20IDE%20CodeStyle%20analyzers.md).

CC @jnm2

Start with the IDE0044 analyzer and compare its existing precondition with the identity-conversion discussion in #85201. Verify the proposed behavior for the C# examples, including struct, reference, pointer, and byref fields, plus constructor-assignment cases, for both C# and VB. Complete the Documentation for IDE CodeStyle analyzers steps when the diagnostic behavior is updated.

Written by the indexing model from the issue text.