Added multiline strings to the Scala syntax highlighter (#1969)

In Scala multiline strings are constructed just like in Python

```
val message = """
This
is
a
message
"""

runtime/syntax/scala.yaml

@@ -7,6 +7,10 @@ rules:
     - type: "\\b(boolean|byte|char|double|float|int|long|new|short|this|transient|void)\\b"
     - statement: "\\b(match|val|var|break|case|catch|continue|default|do|else|finally|for|if|return|switch|throw|try|while)\\b"
     - statement: "\\b(def|object|case|trait|lazy|implicit|abstract|class|extends|with|final|implements|override|import|instanceof|interface|native|package|private|protected|public|static|strictfp|super|synchronized|throws|volatile|sealed)\\b"
+    - constant.string:
+        start: "\"\"\""
+        end: "\"\"\""
+        rules: []
     - constant.string:
         start: "\""
         end: "\""