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/strings/str_/case/camel_case.rs

@@ -28,7 +28,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Convert a string to camelCase"
+        "Convert a string to camelCase."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/strings/str_/case/capitalize.rs

@@ -26,7 +26,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Capitalize first letter of text"
+        "Capitalize first letter of text."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/strings/str_/case/downcase.rs

@@ -26,7 +26,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Make text lowercase"
+        "Make text lowercase."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/strings/str_/case/kebab_case.rs

@@ -28,7 +28,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Convert a string to kebab-case"
+        "Convert a string to kebab-case."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/strings/str_/case/pascal_case.rs

@@ -28,7 +28,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Convert a string to PascalCase"
+        "Convert a string to PascalCase."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/strings/str_/case/screaming_snake_case.rs

@@ -27,7 +27,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Convert a string to SCREAMING_SNAKE_CASE"
+        "Convert a string to SCREAMING_SNAKE_CASE."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/strings/str_/case/snake_case.rs

@@ -27,7 +27,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Convert a string to snake_case"
+        "Convert a string to snake_case."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/strings/str_/case/str_.rs

@@ -20,7 +20,7 @@ impl Command for Str {
     }
 
     fn usage(&self) -> &str {
-        "Various commands for working with string data"
+        "Various commands for working with string data."
     }
 
     fn extra_usage(&self) -> &str {

crates/nu-command/src/strings/str_/case/title_case.rs

@@ -28,7 +28,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Convert a string to Title Case"
+        "Convert a string to Title Case."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/strings/str_/case/upcase.rs

@@ -24,7 +24,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Make text uppercase"
+        "Make text uppercase."
     }
 
     fn search_terms(&self) -> Vec<&str> {