Support highlighting of separator lines between editor and bread… (#136)

Previously the `SEPARATOR_ABOVE_COLOR` and `SEPARATOR_BELOW_COLOR`
editor scheme keys, which were added to the IDE platform code in
JetBrains/intellij-community@dda1191,
were not defined explicitly which makes them inherit color values from
other keys. They have now been explicitly defined using `nord3` as
foreground color to ensure style consistency across IDE version updates.

Closes GH-125

resources/themes/nord.xml

@@ -60,6 +60,8 @@
     <option name="SELECTED_INDENT_GUIDE" value="616e88" />
     <option name="SELECTED_TEARLINE_COLOR" value="434c5e" />
     <option name="SELECTION_BACKGROUND" value="434c5e" />
+    <option name="SEPARATOR_ABOVE_COLOR" value="4c566a" />
+    <option name="SEPARATOR_BELOW_COLOR" value="f000d7" />
     <option name="SOFT_WRAP_SIGN_COLOR" value="4c566a" />
     <option name="TAB_UNDERLINE" value="88c0d0" />
     <option name="TAB_UNDERLINE_INACTIVE" value="616e88" />