feat(guix_shell): Initial implementation (#4397)

* feat(guix_shell): Initial implementation (#3999)

* fix(guix_shell): Change guix nerd font icon to water buffalo emoji

* fix(guix_shell): Added guix_shell entries in preset files

* fix(guix_shell): Moved guix_shell config docs in to the correct place (alphabetically)

docs/config/README.md

@@ -250,6 +250,7 @@ $elixir\
 $elm\
 $erlang\
 $golang\
+$guix_shell\
 $haskell\
 $helm\
 $java\
@@ -1842,6 +1843,39 @@ By default the module will be shown if any of the following conditions are met:
 format = "via [🏎💨 $version](bold cyan) "
 ```
 
+## Guix-shell
+
+The `guix_shell` module shows the [guix-shell](https://guix.gnu.org/manual/devel/en/html_node/Invoking-guix-shell.html) environment.
+The module will be shown when inside a guix-shell environment.
+
+### Options
+
+| Option     | Default                    | Description                                            |
+| ---------- | -------------------------- | ------------------------------------------------------ |
+| `format`   | `'via [$symbol]($style) '` | The format for the module.                             |
+| `symbol`   | `"🐃 "`                     | A format string representing the symbol of guix-shell. |
+| `style`    | `"yellow bold"`            | The style for the module.                              |
+| `disabled` | `false`                    | Disables the `guix_shell` module.                      |
+
+### Variables
+
+| Variable | Example | Description                          |
+| -------- | ------- | ------------------------------------ |
+| symbol   |         | Mirrors the value of option `symbol` |
+| style\*  |         | Mirrors the value of option `style`  |
+
+*: This variable can only be used as a part of a style string
+
+### Example
+
+```toml
+# ~/.config/starship.toml
+
+[guix_shell]
+disabled = true
+format = 'via [🐂](yellow bold) '
+```
+
 ## Haskell
 
 The `haskell` module finds the current selected GHC version and/or the selected Stack snapshot.