Add Japanese installer resources for Firebird 4.0

[tomneko]

Add Japanese installer resources for Firebird 4.0

This pull request adds Japanese language support to the Firebird 4.0 Windows installer.

Changes included:

• Added Japanese language entry to FirebirdInstall.iss
  • Name: ja; MessagesFile: compiler:Languages\Japanese.isl
  • Added Japanese-specific InfoBefore/InfoAfter files
• Added ja/custom_messages_ja.inc with translated installer messages
• Added Japanese documentation files:
  • ja/readme.txt
  • ja/installation_readme.txt
    (These files follow the same structure as other languages and remain in English,
    as the installer documentation is not required to be translated.)
• Updated BuildExecutableInstall.bat to include the ja directory in the language loop
• Added file copy rule for Japanese documentation in the installer script

The structure follows the existing pattern used for other languages (e.g., Russian, German, French), and does not affect any existing installer behavior.

The installer was successfully built and tested on my environment (Windows), and no issues were observed.

Please let me know if any adjustments or additional changes are required.