[PORT] Supporting iterate string by split function

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3328

closes #3279

Now split function can take a single parameter:
split('hello'), new string[] { "h", "e", "l", "l", "o" }

Also if the seperator is a null, it will return same result as the seperator is string.Empty. If both input string and seperator are string.Empty or null, it will return a empty string array.
split('hello',''), new string[] { "h", "e", "l", "l", "o" }
split('',''), new string[] { }
split('hello',nullObj), new string[] { "h", "e", "l", "l", "o" }

If the input string is null, it will return string.Empty regardless the content of the non-empty seperator.
split(nullObj,'e')", new string[] { string.Empty }

Changed projects

• Microsoft.Bot.Expressions
• Microsoft.Bot.Expressions.Tests