Change diagnostic symbols if vim.g.have_nerd_font is true

[glmlm]

Diagnostic symbols on sign column is now displayed graphically when the nerd font is enabled. The reference page is here.

[iton0]

I think this should be a overall comment that users can uncomment if they want this change.

[glmlm]

That might indeed be the case. I will update soon.

[feoh]

Love this. thanks for commenting it out so we can avoid the "You moved my cheese!" crowd :)