Add lines-of-code tags

This is a proposed method for advertising which queries are measuring
the lines of code in a project in a more robust manner than inspecting
the rule id.

Note that the python "LinesOfUserCode" query should _not_ have this
property, as otherwise the results of the two queries will be summed.

java/ql/src/Metrics/Summaries/LinesOfCode.ql

@@ -6,6 +6,7 @@
  *              or comments.
  * @kind metric
  * @tags summary
+ *       lines-of-code
  */
 
 import java