cli: make messages/ part of the configurable --pattern flag

Moves the messages/ path segment from being hardcoded into the export
and import commands to being part of the pattern itself. The default
pattern is now messages/{id}.{lang}.json, giving full control over the
directory structure under the translations output directory.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

docs/tooling/cli/03-commands.md

@@ -447,8 +447,9 @@ Usage: backstage-cli translations export [options]
 
 Options:
   --output <dir>       Output directory for exported messages and manifest (default: "translations")
-  --pattern <pattern>  File path pattern for message files, with {id} and {lang}
-                       placeholders (default: "{id}.{lang}.json")
+  --pattern <pattern>  File path pattern for message files relative to the output
+                       directory, with {id} and {lang} placeholders
+                       (default: "messages/{id}.{lang}.json")
   -h, --help           display help for command
 ```