Make the Nerd Font an optional requirement

Fixes nvim-lua/kickstart.nvim#700

Add a global variable to toggle whether to use a Nerd Font or not:
`vim.g.have_nerd_font` which defaults to false.

Add a mention of Nerd Font to the README

The Nerd Font is used by:
- lazy.nvim (in :Lazy command)
- mini.statusline (git, lsp, filetype)
- Telescope (via nvim-web-devicons)

README.md

@@ -27,6 +27,8 @@ If you are experiencing issues, please make sure you have the latest versions.
 External Requirements:
 - Basic utils: `git`, `make`, `unzip`, C Compiler (`gcc`)
 - [ripgrep](https://github.com/BurntSushi/ripgrep#installation)
+- A [Nerd Font](https://www.nerdfonts.com/): optional, provides various icons
+  - if you have it set `vim.g.have_nerd_font` in `init.lua` to true
 - Language Setup:
   - If want to write Typescript, you need `npm`
   - If want to write Golang, you will need `go`