Add debug mode with slog for logging

[catatsuy]

This pull request introduces changes to the internal/cli/cli.go, internal/slack/client.go, and internal/slack/client_test.go files. The major changes include the addition of a debug mode in the CLI, switching from the standard log package to the structured log package (slog), and updating the tests to reflect these changes.

Changes to the CLI (internal/cli/cli.go):

• Imported the log/slog package for structured logging.
• Added a debugMode boolean variable to the Run function. [1] [2]
• Created a new slog.Logger instance based on the debugMode variable.
• Updated the NewClientForPostFile and NewClient functions to pass the logger instance.

Changes to the Slack client (internal/slack/client.go):

• Replaced the standard log package with the log/slog package.
• Replaced the Logger field in the Client struct with a slog.Logger instance.
• Updated the NewClient and NewClientForPostFile functions to accept a slog.Logger instance. [1] [2]
• Removed the creation of a discard logger in the NewClient and NewClientForPostFile functions.
• Added debug logs to the PostText and PostFile functions. [1] [2]

Changes to the Slack client tests (internal/slack/client_test.go):

• Imported the log/slog package.
• Updated all test functions to create a new slog.Logger instance and pass it to the NewClient and NewClientForPostFile functions. [1] [2] [3] [4] [5] [6] [7] [8] [9]