Allow customizing the font-family for inline completions

It is possible to see this feature with the new Monaspace font family! To test it, download the monaspace font using the following (if on MacOS):

brew tap homebrew/cask-fonts
brew install font-monaspace

Then set the font in the settings using:

"editor.fontFamily": "'Monaspace Neon', monospace",
"editor.inlineSuggest.fontFamily": "'Monaspace Krypton', monospace",

The resulting ghost text looks as follows: