Enable Screen Wake Lock API if possible

[yhatt]

An upcoming Chrome 84 has provided the stable Screen Wake Lock API to prevent device going to sleep. As one of use cases, it is suggested for a web-based presentation app. (https://web.dev/wakelock/#use-cases)

It's a good thing that the HTML slide deck converted by Marp CLI should use this if possible when presenting in fullscreen.

However may not work in the local file and HTTP server mode, the most popular use cases of Marp. According to following condition, Screen Wake Lock API would work only when published the HTML into web.

Like most other powerful web APIs, the Screen Wake Lock API is only available when served over HTTPS.
-- https://web.dev/wakelock/#best-practices