update expression escape

[Danieladu]

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

[boydc2014]

should we throw error when the escape is not valid? if user want \, they should use \\

[boydc2014]

should we throw error when the escape is not valid? if user want \, they should use \\

Actually not, we should not throw, because user will not aware of this error until runtime, we should tolerant that. So your implementation make sense