Use simpler symbols if Nerd Font isn't available

[heathhenley]

From discussion here: #8 (comment)
eg:

it's probably a good idea to check for the supported font and use some alternative simpler symbols

If Hack Nerd Font Mono isn't installed, the icons for text files, folders, binary files, etc render as something like �. Seems like this would involve checking if the font is available, and if not selecting more widely supported alternatives for the icons.

Maybe there could also be a note in "About" or something to hint that you should install nerd font?

[chshersh]

@heathhenley Thanks for creating an issue! I can see several subtasks here:

• Detect if the Nerd Font Mono installed (I have no idea how to do it yet, need to research)
• Find alternative simple icons
• Somehow parametrise the view to dynamically select icons based on the setting
• Display a warning (this probably deserves a separate discussion on, how to display it)

Currently, all icons are written here:

github-tui/lib/tui/widget.ml

Lines 57 to 60 in 71fa359

	let pwd_char = "\u{e5fd}"
	let dir_char = "\u{f4d4}"
	let empty_dir_char = "\u{f413}"
	let file_char = "\u{f4a5}"