Change severity of template instantiation message [backport] (#14526)

* Change severity of template instatiation msg

* Add a test

* Trailing newline argh

nimsuggest/tests/ttempl_inst.nim

@@ -0,0 +1,13 @@
+template foo() =
+  {.warning: "foo".}
+  
+foo()
+
+#[!]#
+discard """
+$nimsuggest --tester $file
+>chk $1
+chk;;skUnknown;;;;Hint;;???;;0;;-1;;"ttempl_inst [Processing]";;0
+chk;;skUnknown;;;;Hint;;$file;;4;;3;;"template/generic instantiation from here";;0
+chk;;skUnknown;;;;Warning;;$file;;2;;11;;"foo [User]";;0
+"""