`[RCS1264]` `roslynator_use_var = when_type_is_obvious` should recognize `StringBuilder`

[RenderMichael]

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Product and Version Used:

NuGet package v. 4.10.0

Steps to Reproduce:

in .editorconfig:

roslynator_use_var = when_type_is_obvious

Code:

internal class Demo
{
    public static string StringBuilderDemo(List<int> entries)
    {
        var sb = new StringBuilder("Entries: ")
            .Append(entries.Count)
            .Append('.');

        return sb.ToString();
    }
}

Actual Behavior:

On the var keyword: RCS1264: Use 'var' or explicit type

Expected Behavior:

The StringBuilder is a well-known type, and this is not an uncommon pattern in my experience.

Much like Parse(string) methods, StringBuilder's Append methods should be recognized as "obviously" returning StringBuilder.