feat(lsp): add goto declaration for FB, DB, and UDT types

- Add declaration provider (gD) that finds definitions for:
  - Functions in .scl files
  - Function Blocks (FB) in .scl files
  - Data Blocks (DB) in .scl and .db files
  - User-Defined Types (UDT) in .scl and .udt files
- Implement project root detection for cross-directory searches
- Add duplicate location filtering to prevent multiple results
- Load UDT types from .udt files in plc_json
- Update documentation with goto declaration feature

src/plc_json.lua

@@ -198,6 +198,21 @@ function M.load_types_from_workspace(root_dir)
     end
   end
 
+  -- Also scan for .udt files
+  local udt_files = scan_files_recursive(root_dir, "*.udt")
+  for _, filepath in ipairs(udt_files) do
+    local content = run_command("cat " .. filepath:gsub(" ", "\\ "))
+    if content then
+      local udt_types = parse_scl_type_file(content)
+      for name, typ in pairs(udt_types) do
+        typ.source = "udt_file"
+        if not types[name] then
+          types[name] = typ
+        end
+      end
+    end
+  end
+
   return types
 end