feat: mimic lualine mode indicator for both fg and bg

[mrolli]

When using highlight set to foreground, actually mimic the exact same
style that the lualine mode indicator has, e.g. setting fg and bg
accordingly. For highlight set to background, only set fg as is.

Fixes #34

[mrolli]

For highlight = 'fg' and being in mode visual, this looks like this then:

As you see, the line indicator and the mode indicator of lualine now look equally the same.

[mawkler]

Your PR seems to change the behaviour of the integration.lualine.highlight option. The intended behaviour of integration.lualine.highlight is to tell Modicator whether to pick the color of the fg or the bg from lualine's highlight for Modicator's mode highlights. It seems that your intention is for this feature to specify which of fg or bg to be set for modicator's highlight. If that is the case it should live in a new option. Perhaps the old one could be renamed to integration.lualine.highlight.pick, and the new one could be integration.lualine.highlight.set, or something similar?

Also, it would be great if you could document this new behaviour in the Default Configuration section, and modify the Lualine Integration section so that the option name of integration.lualine.highlight mentioned in there is correct.