Windows: use $PATHEXT to find executables in path.

is_executable_in_path() searches for executables in $PATH, but on
Windows executable files have extensions available in the environment
var $PATHEXT. This commit changes is_executable_in_path() to append
those extensions to the filename.

This patch diverges from standard Vim, in that Vim only checked for
the given filename if it already has an extensions. This one always
checks for the given filename.

src/nvim/os/fs.c

@@ -146,6 +146,16 @@ static bool is_executable_in_path(const char_u *name, char_u **abspath)
   }
 
   size_t buf_len = STRLEN(name) + STRLEN(path) + 2;
+
+#ifdef WIN32
+  const char *pathext = os_getenv("PATHEXT");
+  if (!pathext) {
+    pathext = ".com;.exe;.bat;.cmd";
+  }
+
+  buf_len += STRLEN(pathext);
+#endif
+
   char_u *buf = xmalloc(buf_len);
 
   // Walk through all entries in $PATH to check if "name" exists there and
@@ -169,6 +179,38 @@ static bool is_executable_in_path(const char_u *name, char_u **abspath)
       return true;
     }
 
+#ifdef WIN32
+    // Try appending file extensions from $PATHEXT to the name.
+    char *buf_end = xstrchrnul((char *)buf, '\0');
+    for (const char *ext = pathext; *ext; ext++) {
+      // Skip the extension if there is no suffix after a '.'.
+      if (ext[0] == '.' && (ext[1] == '\0' || ext[1] == ';')) {
+        *ext++;
+
+        continue;
+      }
+
+      const char *ext_end = xstrchrnul(ext, ENV_SEPCHAR);
+      STRLCPY(buf_end, ext, ext_end - ext + 1);
+
+      if (is_executable(buf)) {
+        // Check if the caller asked for a copy of the path.
+        if (abspath != NULL) {
+          *abspath = save_absolute_path(buf);
+        }
+
+        xfree(buf);
+
+        return true;
+      }
+
+      if (*ext_end != ENV_SEPCHAR) {
+        break;
+      }
+      ext = ext_end;
+    }
+#endif
+
     if (*e != ENV_SEPCHAR) {
       // End of $PATH without finding any executable called name.
       xfree(buf);