Generate const fields for applicable types

[NoahStolk]

Closes #125

This generates const fields for applicable types, for example:

public partial class CustomerType : 
    global::System.IEquatable<CustomerType>, 
    global::System.IComparable, 
    global::System.IComparable<CustomerType> 
{
    // const fields...
    public const System.Int32 NormalValue = 0;
    public const System.Int32 GoldValue = 1;
    public const System.Int32 DiamondValue = 2;

This allows us to write:

public partial class CustomerType
{
    public string GetIcon()
    {
        return Value switch
        {
            NormalValue => "🟢",
            GoldValue => "🟡",
            DiamondValue => "🔵",
            _ => throw new UnreachableException(),
        };
    }
}

instead of:

public partial class CustomerType
{
    public string GetIcon()
    {
        if (this == Normal)
            return "🟢";
    
        if (this == Gold)
            return "🟡";
    
        if (this == Diamond)
            return "🔵";

        throw new UnreachableException();
    }
}

The new generator method ignores types that are not constants, such as:

public record class Planet(string Colour, int CircumferenceInMiles)

It would probably be nice to have a test that explicitly validates that these const fields are only generated for primitives, strings, and decimals.

[SteveDunn]

LGTM! Thanks for the contribution - much appreciated!

[arteny]

it is not working for string underlaying type, getting error:

Severity	Code	Description	Project	File	Line	Suppression State
Error (active)	CS9135	A constant value of type 'ColorStyle' is expected

[NoahStolk]

it is not working for string underlaying type, getting error:

Severity	Code	Description	Project	File	Line	Suppression State
Error (active)	CS9135	A constant value of type 'ColorStyle' is expected

@arteny I've tested this extensively with strings and I'm currently using it in my code. It compiles fine. Are you sure you're not using the members in your switch expressions instead of the new const fields? You can only use the const fields (the ones ending in "Value") in switch expressions.

[arteny]

it is not working for string underlaying type, getting error:

Severity	Code	Description	Project	File	Line	Suppression State
Error (active)	CS9135	A constant value of type 'ColorStyle' is expected

@arteny I've tested this extensively with strings and I'm currently using it in my code. It compiles fine. Are you sure you're not using the members in your switch expressions instead of the new const fields? You can only use the const fields (the ones ending in "Value") in switch expressions.

Ahh, understood now, we need to use other member with Value ending. Looks like it is workint well. Thank you.