Uniformize usage() and extra_usage() message ending for commands helper. (#8268)

# Description

Working on uniformizing the ending messages regarding methods usage()
and extra_usage(). This is related to the issue
https://github.com/nushell/nushell/issues/5066 after discussing it with
@jntrnr

# User-Facing Changes

None.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/env/config/config_.rs

@@ -20,7 +20,7 @@ impl Command for ConfigMeta {
     }
 
     fn usage(&self) -> &str {
-        "Edit nushell configuration files"
+        "Edit nushell configuration files."
     }
 
     fn extra_usage(&self) -> &str {

crates/nu-command/src/env/config/config_env.rs

@@ -23,7 +23,7 @@ impl Command for ConfigEnv {
     }
 
     fn usage(&self) -> &str {
-        "Edit nu environment configurations"
+        "Edit nu environment configurations."
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/env/config/config_nu.rs

@@ -23,7 +23,7 @@ impl Command for ConfigNu {
     }
 
     fn usage(&self) -> &str {
-        "Edit nu configurations"
+        "Edit nu configurations."
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/env/config/config_reset.rs

@@ -26,7 +26,7 @@ impl Command for ConfigReset {
     }
 
     fn usage(&self) -> &str {
-        "Reset nushell environment configurations to default, and saves old config files in the config location as oldconfig.nu and oldenv.nu"
+        "Reset nushell environment configurations to default, and saves old config files in the config location as oldconfig.nu and oldenv.nu."
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/env/env_command.rs

@@ -14,7 +14,7 @@ impl Command for Env {
     }
 
     fn usage(&self) -> &str {
-        "Display current environment variables"
+        "Display current environment variables."
     }
 
     fn signature(&self) -> nu_protocol::Signature {