How to display hidden files with Visual Studio Code

No setting, but you can hit cmdshift. in the open file dialog to see hidden files.

(Source)

Under Linux you can right click on files in the file selection window and check the "Show Hidden Files" checkbox.

Here's a screenshot from Ubuntu 20.04.1 LTS, VS Code v 1.52.1 after right clicking on the header.php file with "Show Hidden Files" option enabled:

go to file -> preferences -> settings

and in the search bar, search for "files.exclude"

then delete whatever json setting that you don't want to hide that specific file