Replace custom highlighting with plugin RRethy/vim-illuminate

[denius]

This is a PR for #1253, it replaces the word highlighting functions with a plugin that has more customization options.

The main reason for this PR is that the highlighting functionality in kickstart.nvim is incompatible with the use of highlighting plugins, as there is no way to disable the built-in highlighting other than commenting it out entirely.

The configuration is completely taken from the plugin page RRethy/vim-illuminate.

[iton0]

I think introducing a plugin for cursor word highlighting doesn't fit in with kickstart. However I think given the incompatibility issue, it would be beneficial to update the comments for this code. This way new users can be made aware and either keep the default or comment out and add whichever highlighting plugins in their own configs.

[feoh]

@denius Thank you for your PR. I know I suggested it, but I think @iton0 is right and the way to go here is to just suggest that users have the option to use this in comments.

[denius]

I realized that it is better to do without an extra plugin if possible.