Replace `NvimWebDeviconsHiTest` command with `checkhealth`

[gegoune]

Discussed in #552

^{Originally posted by gegoune January 26, 2025}
@alex-courtis what do you think about replacing NvimWebDeviconsHiTest to :checkhealth nvim-web-devicons? Can keep the same output but will get whatever benefits :che gives us for free.