fix: add missing OilExecutableHidden highlight group

Problem: the rendering code constructs "OilExecutable" .. "Hidden" for
hidden executables, but that group was never defined. Hidden executables
silently lose all highlighting instead of being dimmed like every other
hidden entry type.

Solution: add OilExecutableHidden linked to OilHidden, consistent with
OilFileHidden, OilDirHidden, and all other hidden variants.

Based on: stevearc/oil.nvim#698

lua/oil/init.lua

@@ -928,6 +928,11 @@ M._get_highlights = function()
       link = "DiagnosticOk",
       desc = "Executable files in an oil buffer",
     },
+    {
+      name = "OilExecutableHidden",
+      link = "OilHidden",
+      desc = "Hidden executable files in an oil buffer",
+    },
     {
       name = "OilCreate",
       link = "DiagnosticInfo",