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/platform/ansi/gradient.rs

@@ -54,7 +54,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Add a color gradient (using ANSI color codes) to the given string"
+        "Add a color gradient (using ANSI color codes) to the given string."
     }
 
     fn run(

crates/nu-command/src/platform/ansi/link.rs

@@ -45,7 +45,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Add a link (using OSC 8 escape sequence) to the given string"
+        "Add a link (using OSC 8 escape sequence) to the given string."
     }
 
     fn run(

crates/nu-command/src/platform/ansi/strip.rs

@@ -25,7 +25,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Strip ANSI escape sequences from a string"
+        "Strip ANSI escape sequences from a string."
     }
 
     fn run(

crates/nu-command/src/platform/reedline_commands/keybindings.rs

@@ -20,7 +20,7 @@ impl Command for Keybindings {
     }
 
     fn usage(&self) -> &str {
-        "Keybindings related commands"
+        "Keybindings related commands."
     }
 
     fn extra_usage(&self) -> &str {

crates/nu-command/src/platform/reedline_commands/keybindings_default.rs

@@ -20,7 +20,7 @@ impl Command for KeybindingsDefault {
     }
 
     fn usage(&self) -> &str {
-        "List default keybindings"
+        "List default keybindings."
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/platform/reedline_commands/keybindings_list.rs

@@ -28,7 +28,7 @@ impl Command for KeybindingsList {
     }
 
     fn usage(&self) -> &str {
-        "List available options that can be used to create keybindings"
+        "List available options that can be used to create keybindings."
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/platform/term_size.rs

@@ -14,7 +14,7 @@ impl Command for TermSize {
     }
 
     fn usage(&self) -> &str {
-        "Returns a record containing the number of columns (width) and rows (height) of the terminal"
+        "Returns a record containing the number of columns (width) and rows (height) of the terminal."
     }
 
     fn signature(&self) -> Signature {