More of StringStream now works at compile time (#12284)

* readLine of StringStream now works at compile time
* Work for js backend only at compile time
* readAll now works at CT
* readAll works in js ct now
* Add test

tests/stdlib/tstreams3.nim

@@ -1,5 +1,10 @@
 discard """
   output: "threw exception"
+  nimout: '''
+I
+AM
+GROOT
+'''
 """
 import streams
 
@@ -7,3 +12,9 @@ try:
   var fs = openFileStream("shouldneverexist.txt")
 except IoError:
   echo "threw exception"
+
+static:
+  var s = newStringStream("I\nAM\nGROOT")
+  for line in s.lines:
+    echo line
+  s.close