Typo fixes

[NewDawn0]

Hey Keyitdev 👋

This PR brings a few small but nice clean-ups to the Astronaut theme collection:

1. Consistent kebab-case everywhere

• Before: mix of snake_case, and kebab-case
• All theme folder names and references are now consistently kebab-case
  (e.g. purple_leaves → purple-leaves, post-apocalyptic_hacker → post-apocalyptic-hacker, etc.)
• This matches the overwhelming majority of SDDM themes on the web and makes the repo easier to package (especially on Nix/NixOS).

2. Fixed various typos and small grammar issues

• README, theme descriptions, comments, etc.
• Purely cosmetic, no functional changes.

3. Added Nix flake support

• Now all the themes are build-able using the Nix package manager.

  nix build github.com:Keytidev/sddm-astronaut-theme#sddm-<theme-name>-theme

• This includes support for overlays whereby they are available under the sddm-themes namespace. However do note that this does not install the themes into /usr/share/sddm/themes.

# In your flake.nix
# 1. Add it to your inputs
{
  inputs.sddm-themes.url = "github.com:Keytidev/sddm-astronaut-theme";
  inputs.sddm-themes.nixpkgs.follows = "nixpkgs";
}
# 2. Add the overlay
[ # Other overlays ...
  inputs.sddm-themes.overlays.default
]
# 3. In your system packages
environment.systemPackages = [ pkgs.sddm-themes.sddm-astronaut-theme ];