Implement MissingHelper feature

[oformaniuk]

Implementation for a missing feature mentioned in Handlebars-Net/issues/350.

The are two options to configure the feature:

• Registering helperMissing helper like described in Handlebars documentation:

Handlebars.RegisterHelper("helperMissing", (writer, context, arguments) =>
{
    var helperName = arguments.Last().ToString();
    writer.WriteSafeString($"Helper missing: {helperName}");
});

Or

Handlebars.RegisterHelper("helperMissing", (context, arguments) =>
{
    var helperName = arguments.Last().ToString();
    return $"Helper missing: {helperName}";
});

• Registering blockHelperMissing helper like described in Handlebars documentation:

Handlebars.RegisterHelper("blockHelperMissing", (writer, options, context, arguments) =>
{
    var helperName = options.GetValue<string>("name");
    writer.WriteSafeString($"Helper missing: {helperName}");
});

• Registering via feature:

Handlebars.Configuration.RegisterMissingHelperHook(
    helperMissing: (context, arguments) =>
    {
        var name = arguments.Last().ToString();
        return string.Format(format, name.Trim('[', ']'));
    },
    blockHelperMissing: (writer, options, context, arguments) =>
    {
        var helperName = options.GetValue<string>("name");
        writer.WriteSafeString($"Helper missing: {helperName}");
    }
);

[oformaniuk]

TODO: implement blockHelperMissing properly

[oformaniuk]

Issue Handlebars-Net/issues/354 discovered. However the problem does not reproduce in current version.

[sonarqubecloud]

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