[PORT] update expression escape

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3580

close: #3510
update Expression escape policy to make expression achieve higher robustness.

the new strategy of expression evaluation about escape characters is as below:

• in any case, \n, \r, \t, \\ would be unescaped.
• In double quotation mark string, \" would be unescaped. There is no need to escape `and ' in double quotation mark string
• In single quotation marks string, \' would be unescaped. There is no need to escape ` and " in single quotation mark string
• in string interpolation mode, \` and \$ would be unescaped
• other characters that have \ prefix, would leave it as is
  samples:

expression	result	description
`hi\``	hi`	support ` escape in string interpolation
`\${a}`	${a}	escape $ in string interpolation
"ab\"cd"	ab"cd	escape " in double quot string
'ab\'cd'	ab'cd	escape ' in single quot string
"ab`cd"	ab`cd	no need to escape ` in normal string(' or ")
"ab\ncd"	ab [newline] cd	newline would be unescaped
`hi\y`	hi\y	unrecognized escape character would not be unescaped
"ab\ycd"	ab\ycd	unrecognized escape character would not be unescaped

Changed projects

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

[LG,R9]