Add support of escaping backslash and backtick to String.raw (#3736)

Template literals already supports escaping backslash and backtick,
this commit enables escaping this in String.raw.
It also fixes invalid behaviour of String.raw when using
new line character.

JerryScript-DCO-1.0-Signed-off-by: Rafal Walczyna r.walczyna@samsung.com

tests/jerry/es2015/string-raw.js

@@ -62,3 +62,10 @@ var raw = new Proxy({length: 2, 0: '', 1: ''}, { get: function(o, k) { get.push(
 var p = new Proxy({raw: raw}, { get: function(o, k) { get.push(k); return o[k]; }});
 String.raw(p);
 assert(get + '' === "raw,length,0,1");
+
+assert(String.raw`\\` == "\\\\")
+assert(String.raw`\`` == "\\`")
+assert(String.raw`\
+\
+` == "\\\n\\\n")
+assert(String.raw`\
` == "\\\u2029")