fix(sonokai): add primary cursor colors

[pcrockett]

Many themes, including my personal favorite sonokai, specify the ui.cursor.* settings but don't yet have ui.cursor.primary.* settings configured. This causes primary cursor colors not to change correctly when switching to visual mode (for example).

I've fixed it for sonokai at least in this PR. A similar issue has been previously discussed here.

This strikes me as a band-aid, however. I think the issue should be solved more fundamentally, though I'm unsure which approach the Helix project would prefer:

• Would it be useful to make a similar change for all the themes that specify a cursor color without specifying a primary? Like this commit?
  • I'd be happy to adjust this PR to make that change, though I wouldn't want to test every single theme myself.
• Or would it be a better approach to change theme logic to make the primary cursor color default to the general cursor color setting, so you don't have to duplicate the settings in all themes?