feat: add highlight for executable files

Problem: executable files were visually indistinguishable from regular
files in oil buffers.

Solution: add OilExecutable highlight group (linked to DiagnosticOk)
that detects executables via Unix permission bits (S_IXUSR|S_IXGRP|
S_IXOTH) and Windows executable extensions (.exe, .bat, .cmd, .com,
.ps1). Applied to both regular files and symlink targets.

Based on: stevearc/oil.nvim#698
Closes stevearc/oil.nvim#679

lua/oil/init.lua

@@ -923,6 +923,11 @@ M._get_highlights = function()
       link = "OilHidden",
       desc = "Hidden normal files in an oil buffer",
     },
+    {
+      name = "OilExecutable",
+      link = "DiagnosticOk",
+      desc = "Executable files in an oil buffer",
+    },
     {
       name = "OilCreate",
       link = "DiagnosticInfo",