feat(highlight): implement CurSearch highlight

Adds a `CurSearch` highlight group to highlight the current search result under the cursor.

src/nvim/highlight_defs.h

@@ -63,6 +63,7 @@ typedef enum {
   HLF_E,          // error messages
   HLF_I,          // incremental search
   HLF_L,          // last search string
+  HLF_LC,         // current search match
   HLF_M,          // "--More--" message
   HLF_CM,         // Mode (e.g., "-- INSERT --")
   HLF_N,          // line number for ":number" and ":#" commands
@@ -123,6 +124,7 @@ EXTERN const char *hlf_names[] INIT(= {
   [HLF_E] = "ErrorMsg",
   [HLF_I] = "IncSearch",
   [HLF_L] = "Search",
+  [HLF_LC] = "CurSearch",
   [HLF_M] = "MoreMsg",
   [HLF_CM] = "ModeMsg",
   [HLF_N] = "LineNr",