Support highlighting of line number on caret row (#133)

The line number on the current caret row was previously highlighted with
the same color (`nord3`) like all other line numbers.
This has been changed to use `nord4` instead for lines with active
caret(s) by adding support for the `LINE_NUMBER_ON_CARET_ROW_COLOR`
editor scheme key added to the IDE platform code in
JetBrains/intellij-community@8641fed.

Closes GH-122

resources/themes/nord.xml

@@ -69,6 +69,7 @@
     <option name="VCS_ANNOTATIONS_COLOR_5" value="3b4252" />
     <option name="WHITESPACES" value="4c566a" />
     <option name="WHITESPACES_MODIFIED_LINES_COLOR" value="ebcb8b" />
+    <option name="LINE_NUMBER_ON_CARET_ROW_COLOR" value="d8dee9" />
   </colors>
   <attributes>
     <option name="ABSTRACT_CLASS_NAME_ATTRIBUTES">