[PORT] Add support of multi line definition of expression in LG

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3589

fixes: #3466
Now user can write there expressions in multiple lines, like:
# definition2
- this is book list: ${join(createArray("Ender's Game",
"Dune")
, ", ")}

LG will treat all the content after ${ as an expression until reaches a }.

Changed projects

• Microsoft.Bot.Builder.LanguageGeneration
• AdaptiveExpressions.Tests
• Microsoft.Bot.Builder.LanguageGeneration.Tests

[cosmicshuai]

#1921
JS done first