[PORT] Remove unnecessary restrication on memory path

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3597

Loose a restriction on path introduced in microsoft/botbuilder-dotnet#3226.

To support variable path like "$index"

Changed projects

• AdaptiveExpressions
• AdaptiveExpressions.Tests

[customer-reported]