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/deprecated/export_old_alias.rs

@@ -11,7 +11,7 @@ impl Command for ExportOldAlias {
     }
 
     fn usage(&self) -> &str {
-        "Define an alias and export it from a module"
+        "Define an alias and export it from a module."
     }
 
     fn signature(&self) -> nu_protocol::Signature {

crates/nu-command/src/deprecated/hash_base64.rs

@@ -15,7 +15,7 @@ impl Command for HashBase64 {
     }
 
     fn usage(&self) -> &str {
-        "Deprecated command"
+        "Deprecated command."
     }
 
     fn run(

crates/nu-command/src/deprecated/lpad.rs

@@ -16,7 +16,7 @@ impl Command for LPadDeprecated {
     }
 
     fn usage(&self) -> &str {
-        "Deprecated command"
+        "Deprecated command."
     }
 
     fn run(

crates/nu-command/src/deprecated/math_eval.rs

@@ -21,7 +21,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Deprecated command"
+        "Deprecated command."
     }
 
     fn run(

crates/nu-command/src/deprecated/old_alias.rs

@@ -13,7 +13,7 @@ impl Command for OldAlias {
     }
 
     fn usage(&self) -> &str {
-        "Alias a command (with optional flags) to a new name"
+        "Alias a command (with optional flags) to a new name."
     }
 
     fn signature(&self) -> nu_protocol::Signature {

crates/nu-command/src/deprecated/rpad.rs

@@ -16,7 +16,7 @@ impl Command for RPadDeprecated {
     }
 
     fn usage(&self) -> &str {
-        "Deprecated command"
+        "Deprecated command."
     }
 
     fn run(

crates/nu-command/src/deprecated/str_datetime.rs

@@ -17,7 +17,7 @@ impl Command for StrDatetimeDeprecated {
     }
 
     fn usage(&self) -> &str {
-        "Deprecated command"
+        "Deprecated command."
     }
 
     fn run(

crates/nu-command/src/deprecated/str_decimal.rs

@@ -17,7 +17,7 @@ impl Command for StrDecimalDeprecated {
     }
 
     fn usage(&self) -> &str {
-        "Deprecated command"
+        "Deprecated command."
     }
 
     fn run(

crates/nu-command/src/deprecated/str_find_replace.rs

@@ -17,7 +17,7 @@ impl Command for StrFindReplaceDeprecated {
     }
 
     fn usage(&self) -> &str {
-        "Deprecated command"
+        "Deprecated command."
     }
 
     fn run(

crates/nu-command/src/deprecated/str_int.rs

@@ -17,7 +17,7 @@ impl Command for StrIntDeprecated {
     }
 
     fn usage(&self) -> &str {
-        "Deprecated command"
+        "Deprecated command."
     }
 
     fn run(