Allow user to optionally override the hl group of suggestions

[JasonBoyett]

I think this would be a nice feature and that some users would appreciate the added customization. I personally set the Comment hl group in my config to be italic and a specific color. I don't want my copilot suggestions to be the same as my comments. I was able to solve this issue in my own config by overriding the CopilotSuggestion hl group but I only figured this out by looking through the code. If providing it as a configuration option doesn't seem rite then perhaps a quick mention of how to set your suggestion styles in the docs could be useful.

I'm sure I could implement this myself and would be happy to do so.