feat(shlvl): Add shlvl module (#1385)

* initial commit of support for shlvl

* documentation for shlvl

* use a symbol instead

* test coverage for shlvl

* actually disable when the config says to

* fix docs

* tweak defaults some

* refactor from pr comments

* redisable

* return early to avoid indenting

* make default suffix empty space

* fixing tests after suffix change

* updating docs for format

* making shlvl compatible with formatting

* adding variables table for shlvl

* removing extra line

* doc clarity

docs/config/README.md

@@ -178,6 +178,7 @@ format = """
 
 $username\
 $hostname\
+$shlvl\
 $kubernetes\
 $directory\
 $git_branch\
@@ -1985,6 +1986,42 @@ The module will be shown if any of the following conditions are met:
 format = "via [⚙️ $version](red bold)"
 ```
 
+## SHLVL
+
+The `shlvl` module shows the current SHLVL ("shell level") environment variable, if it is
+set to a number and meets or exceeds the specified threshold.
+
+### Options
+
+| Variable    | Default                      | Description                                      |
+| ----------- | ---------------------------- | ------------------------------------------------ |
+| `threshold` | `2`                          | Display threshold.                               |
+| `format`    | `"[$symbol$shlvl]($style) "` | The format for the module.                       |
+| `symbol`    | `"↕️ "`                       | The symbol used to represent the SHLVL.          |
+| `style`     | `"bold yellow"`              | The style for the module.                        |
+| `disabled`  | `true`                       | Disables the `shlvl` module.                     |
+
+### Variables
+
+| Variable | Example   | Description                          |
+| -------- | --------- | ------------------------------------ |
+| shlvl    | `3`       | The current value of SHLVL           |
+| symbol   |           | Mirrors the value of option `symbol` |
+| style\*  |           | Mirrors the value of option `style`  |
+
+\*: This variable can only be used as a part of a style string
+
+### Example
+
+```toml
+# ~/.config/starship.toml
+
+[shlvl]
+disabled = false
+format = "$shlvl level(s) down"
+threshold = 3
+```
+
 ## Singularity
 
 The `singularity` module shows the current singularity image, if inside a container