StringStream & more stdlib modules support for JS/NimScript (#14095)

* StringStream & more stdlib modules support for JS/NimScript

* change back pegs test in line with #14134

tests/js/tstreams.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''
+I
+AM
+GROOT
+'''
+"""
+
+import streams
+
+var s = newStringStream("I\nAM\nGROOT")
+doAssert s.peekStr(1) == "I"
+doAssert s.peekChar() == 'I'
+for line in s.lines:
+  echo line
+s.close
+
+var s2 = newStringStream("abc")
+doAssert s2.readAll == "abc"
+s2.write("def")
+doAssert s2.data == "abcdef"
+s2.close