Add raw string literals

Also document + test multi line literals.

test/language/string/literals.wren

@@ -3,3 +3,28 @@ System.print("a string") // expect: a string
 
 // Non-ASCII.
 System.print("A~¶Þॐஃ") // expect: A~¶Þॐஃ
+
+// Raw strings.
+System.print("""A raw string""") // expect: A raw string
+
+var long = "
+  A
+  multi line
+  regular string
+"
+System.print(long) // expect: 
+                   // expect:   A
+                   // expect:   multi line
+                   // expect:   regular string
+                   // expect: 
+
+var raw = """
+  A if*(<invalid>)*
+  multi line /{}()
+  raw string [\]/
+  "json": "value"
+"""
+System.print(raw) // expect:   A if*(<invalid>)*
+                  // expect:   multi line /{}()
+                  // expect:   raw string [\]/
+                  // expect:   "json": "value"

test/language/string/unterminated_raw.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"""this string has no close quote