Added candidate naming convention to the style guide (nushell#1601)

Author: vyadh

book/custom_commands.md

@@ -267,10 +267,11 @@ It's common practice in Nushell to separate the words of the command with `-` fo
 ::: tip
 Because `def` is a parser keyword, the command name must be known at parse time. This means that command names may not be a variable or constant. For example, the following is _not allowed_:
 
-````nu
+```nu
 let name = "foo"
 def $name [] { foo }
 :::
+```
 
 ### Subcommands
 
@@ -280,7 +281,7 @@ You can also define subcommands of commands using a space. For example, if we wa
 def "str mycommand" [] {
   "hello"
 }
-````
+```
 
 Now we can call our custom command as if it were a built-in subcommand of [`str`](/commands/docs/str.md):
 

book/style_guide.md

@@ -180,6 +180,141 @@ let selectedProfile = (
 })
 ```
 
+## Naming Convention
+
+### Abbreviations and Acronyms
+
+**It's recommended** to use full concise words over abbreviations and acronyms, unless they are well-known and/or
+commonly used.
+
+Correct:
+
+```nu
+query-user --id 123
+
+$user.name | str downcase
+```
+
+Incorrect:
+
+```nu
+qry-usr --id 123
+
+$user.name | string downcase
+```
+
+### Case
+
+#### Commands
+
+**It's recommended to** use kebab-case for command names with multiple words.
+
+Correct:
+
+```nu
+fetch-user --id 123
+```
+
+Incorrect:
+
+```nu
+fetch_user --id 123
+fetchUser --id 123
+```
+
+See also [Naming Commands](custom_commands.md#naming-commands).
+
+#### Sub-Commands
+
+Sub commands are commands that are logically grouped under a parent command and separated by a space.
+**It's recommended to** use kebab-case for the sub-command name.
+
+Correct:
+
+```nu
+date now
+
+date list-timezone
+
+def "login basic-auth" [username: string password: string] {
+    # ...
+}
+```
+
+See also [Naming Sub-Commands](custom_commands.md#subcommands).
+
+#### Flags
+
+**It's recommended to** use kebab-case for flag names.
+
+Correct:
+
+```nu
+def greet [name: string, --all-caps] {
+    # ...
+}
+```
+
+Incorrect:
+
+```nu
+def greet [name: string, --all_caps] {
+    # ...
+}
+```
+
+::: tip
+Notice that the name used to access the flag is accessed by replacing the dash with an underscore in the resulting
+variable name.
+
+See [Flags](custom_commands.md#flags).
+:::
+
+#### Variables and Command Parameters
+
+**It's recommended to** use snake_case for variable names, including command parameters.
+
+Correct:
+
+```nu
+let user_id = 123
+
+def fetch-user [user_id: int] {
+  # ...
+}
+```
+
+Incorrect:
+
+```nu
+let user-id = 123
+let userId = 123
+
+def fetch-user [user-id: int] {
+  # ...
+}
+```
+
+#### Environment Variables
+
+**It's recommended to** use SCREAMING_SNAKE_CASE for environment variable names.
+
+Correct:
+
+```nu
+$env.ENVIRONMENT_CODE = "prod"
+
+$env.APP_VERSION = "1.0.0"
+```
+
+Incorrect:
+
+```nu
+$env.ENVIRONMENT-CODE = "prod"
+
+$env.app_version = "1.0.0"
+```
+
 ## Options and Parameters of Custom Commands
 
 - **It's recommended to** keep count of all positional parameters less than or equal to 2, for remaining inputs use options. Assume that command can expect source and destination parameter, like `mv`: source and target file or directory.