[PORT] [PORT]Maintain the high priority and independence of multiline

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3944

close #3885
poer from microsoft/botbuilder-js#2221
In such case:

The heading "#" would be treated as the label of a beginning of a template, it is incorrect in multi-line mode.

This fix would make the multi-line mode more independent and has higher priority to handler such cases.

Changed projects

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

[LG,R10]