feat: Add the git_metrics module (#2827)

This PR adds a new module named git_metrics. It shows the added/deleted lines in the current git repository following the format: "[+$added_lines]($added_style) [-$deleted_lines]($deleted_style)".

docs/config/README.md

@@ -194,6 +194,7 @@ $vcsh\
 $git_branch\
 $git_commit\
 $git_state\
+$git_metrics\
 $git_status\
 $hg_branch\
 $docker_context\
@@ -1232,6 +1233,48 @@ format = '[\($state( $progress_current of $progress_total)\)]($style) '
 cherry_pick = "[🍒 PICKING](bold red)"
 ```
 
+## Git Metrics
+
+The `git_metrics` module will show the number of added and deleted lines in
+the current git repository.
+
+::: tip
+
+This module is disabled by default.
+To enable it, set `disabled` to `false` in your configuration file.
+
+:::
+
+### Options
+
+| Option                    | Default                                                               | Description                            |
+| ------------------------- | --------------------------------------------------------------------  | ---------------------------------------|
+| `added_style`             | `"bold green"`                                                        | The style for the added count.         |
+| `deleted_style`           | `"bold red"`                                                          | The style for the deleted count.       |
+| `format`                  | `'[+$added]($added_style) [-$deleted]($deleted_style) '`              | The format for the module.             |
+| `disabled`                | `true`                                                                | Disables the `git_metrics` module.     |
+
+### Variables
+
+| Variable         | Example    | Description                                 |
+| ---------------- | ---------- | -----------------------------------         |
+| added            | `1`        | The current number of added lines           |
+| deleted          | `2`        | The current number of deleted lines         |
+| added_style\*    |            | Mirrors the value of option `added_style`   |
+| deleted_style\*  |            | Mirrors the value of option `deleted_style` |
+
+\*: This variable can only be used as a part of a style string
+
+### Example
+
+```toml
+# ~/.config/starship.toml
+
+[git_metrics]
+added_style = "bold blue"
+format = '[+$added]($added_style)/[-$deleted]($deleted_style) '
+```
+
 ## Git Status
 
 The `git_status` module shows symbols representing the state of the repo in your