Declare HTML5 scripts and styles support for better compliance with W3C validator

[audrasjb]

Hi,

Given this theme is written in HTML5, it should declare HTML5 support for styles and script to avoid using type attributes in styles and scripts.

Otherwise, W3C validator will throw a warning because of the type element.

This option was introduced in WordPress 5.3. For reference, see:

• https://make.wordpress.org/core/2019/10/15/miscellaneous-developer-focused-changes-in-5-3/
• https://core.trac.wordpress.org/ticket/42804#comment:32