update LG escape

[Danieladu]

close: #3549
update LG escape policy to make LG achieve higher robustness.

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

• in any case, \n, \r, \t, \\ would be unescaped.
• in normal text condition, \$ would be unescaped to $
• in structured value condition, \| would be unescaped to |
• other characters that have \ prefix, would leave it as is

Examples

Simple samples

lg template	result
ab\ncd	ab [newline] cd
ab\${cd}	ab${cd}
ab\ycd	ab\ycd
ab\"cd	ab\"cd
ab\'cd	ab\'cd

Structure LG sample

# template
[struct
   list=a|b\|c
]  

would get:

{
    "lgType": "struct",
    "list":[
        "a",
        "b|c"
    ]
}

json sample

# template
- ${json(cardjson()).text}

# cardjson
- ``` {
    "text":"Your meeting about \"Adaptive Card design session\""
}```

would get the string result -> Your meeting about "Adaptive Card design session"

fromFile sample

# template
- ${json(fromFile('a.txt')).text}

and the content of 1.txt is

{
    "text":"Your meeting about \"Adaptive Card design session\""
}

would get the same string result -> Your meeting about "Adaptive Card design session"

[boydc2014]

Can you add some test case around \\| in structured lg