Add a new API to initialize an ImmutableDictionary in a single expression/invocation without first creating a mutable dictionary

[Bartleby2718]

My understanding is that the most efficient way to initialize an ImmutableDictionary<TKey, TValue> is using the builder pattern:

var builder = ImmutableDictionary.CreateBuilder<string, int>();
builder["a"] = 1;
builder["b"] = 2;
var immutableDictionary = builder.ToImmutable();

However, this is verbose and clunky. I don't think there's a built-in API to do this in a single expression or invocation. As such, you cannot initialize an ImmutableDictionary<TKey, TValue> in an expression-bodied member. Of course, you could create a mutable dictionary first and then convert it:

// option 1: ImmutableDictionary.CreateRange
private static readonly ImmutableDictionary<string, int> MyImmutableDictionary1 = ImmutableDictionary.CreateRange(new Dictionary<string, int>
{
    { "a", 1 },
    { "b", 2 }
});

// option 2: ToImmutableDictionary
private static readonly ImmutableDictionary<string, int> MyImmutableDictionary2 = new Dictionary<string, int>
{
    { "a", 1 },
    { "b", 2 }
}.ToImmutableDictionary();

but I think it would be great if this could be done without creating a mutable dictionary.

I'm indifferent about what syntax is used to initialize that, although I did notice that the indexer is readonly and therefore cannot be used:

[HaloFour]

See: #8658

That should enable:

ImmutableDictionary<string, int> MyImmutableDictionary1 = ["a": 1, "b": 2];

[huoyaoyuan]

Also FWIW, ImmutableDictionary is usually not the optimal (correct) type to use. Check FrozenDictionary instead.