Create get_installed_themes.lua

[Subarctic2796]

Hi (I am new to git, so please bare with me)

I made a new file as I didn't know where to put this but I am happy to change it.

Basically I am returning a lua table that has 2 properties.

1. all_installed_themes: this gets all of the currently installed themes from vim itself so you don't have to manually enter the name of all the themes.
2. default_vim_themes: this is a list of all of the default themes installed in neovim when you first download nvim

I think it is good to have this as you can choose to filter out the default installed themes later.

I made a simple theme picker using telescope that I would be happy to share with you if you want to see it