[PORT] [LG] treat everything after first '=' as value for the property

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3341

close: #3339

Originally, if you want to input '=' in the structure value, an escape should be added. like:

# welcome(name)
[Activity
  text = hi, name \= @{name}
]

But now, we just treat the first equals mark as the key/value separator, so, we can write is as:

# welcome(name)
[Activity
  text = hi, name = @{name}
]

there is no need to write the backslash.

Changed projects

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