[PORT] Optimize performance of LG evaluation

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3327

close: #3297

Today, every time a free LG text like "@{welcome}" is evaluated by creating a new "anonymous template" to wrap this free text, and create a temporary LG file with the new content = original content + new anonymous template, for evaluation.

This is not performance friendly. In this pr, we just parse the additional part, so that there is no need to reparse the original content.

Changed projects

• Microsoft.Bot.Builder.LanguageGeneration
• Microsoft.Bot.Expressions.Properties
• Microsoft.Bot.Builder.Dialogs.Adaptive.Templates.Tests

[Danieladu]

combine with #1712