CSS: Include `show`, `hide` & `toggle` methods in the jQuery slim build

CSS: Include `show`, `hide` & `toggle` methods in the jQuery slim build

The `show()`, `hide()` & `toggle()` methods were included in the 3.x jQuery
slim build. The jQuery master build accidentally started to exclude them as
they were only imported in the effects module and the new Rollup-based build
system follows the module dependency graph when excluding modules.

To resolve the issue, import the `css/showHide.js` file directly in the main
`jquery.js` file.

Closes gh-4704
Ref jquery/jquery-migrate#346