lib/plugins/mkVimPlugin: Support lazyloading

[saygo-png]

Closes: #3313

This implementation uses a loop to set the globals. Since this is per plugin many of these loops might be created. I don't know if this will affect performance and I don't know how to benchmark this.

I tested this with vimVisualMulti and rainbow, both plugins can be successfully lazyloaded.

With this change all 400+ plugin modules we have will support lazyloading!

[GaetanLepage]

Does delaying setting global variables actually corresponds to "lazy loading"?
No (plugin) code is really executed when one runs vim.g["foo_bar"] = true, correct?

[saygo-png]

No (plugin) code is really executed when one runs vim.g["foo_bar"] = true, correct?

Yes

Does delaying setting global variables actually corresponds to "lazy loading"?

As i understand lazyloading is not just loading the plugin lazily, it's also about loading it's config lazily. We do this for Lua plugins, and it's done in lz-n docs for vimscript ones. Look at vim-startuptime here https://github.com/lumen-oss/lz.n?tab=readme-ov-file#examples (Though as i'm writing this i realise they reccomend using before instead of after for vimscript plugins)

[saygo-png]

Sorry, missed a big issue. Previous code always generated Lua even if settings was empty or set to things that filter out. The code I pushed makes it work properly but it looks horrible. Is there some better way to do this?