Move theme setup into after/plugin for portability

Currently the theme configuration is done regardless of the theme actually being installed.

https://gitlab.com/4w/neovim-config/-/blob/ed023a56715a3783c75eba8f930411868c81ae4a/init.lua#L34-37

The theme configuration should be placed in ./after/plugin/neovim-tango-colors.lua and guarded accordingly (see #9 (closed)) to prevent theme-related errors when porting the configuration and Packer sync wasn't done yet.