use new api for uploading file

[catatsuy]

refs: #148

This pull request introduces significant changes to the Slack API implementation, including updates to the file upload process and changes to the command line interface (CLI) and configuration files. The most important changes are the deprecation of the files.upload API, the introduction of new APIs for file uploads, and the replacement of the channel parameter with channel-id for file uploads.

API Changes:

• CHANGELOG.md: The files.upload API is now deprecated and replaced with files.getUploadURLExternal and files.completeUploadExternal. The channel parameter for file uploads has been replaced with channel-id. The -filetype option has been modified and replaced with -snippet-type.

CLI Changes:

• internal/cli/cli.go: The channel parameter in the CLI has been replaced with channel-id. The -filetype option has been marked as compatible and replaced with -snippet-type. The uploadSnippet function now requires a channel-id and snippet-type instead of a channel and filetype. [1] [2] [3] [4]

Configuration Changes:

• README.md, internal/config/config.go, internal/config/config_test.go: The configuration files and tests have been updated to replace the channel parameter with channel-id. The NOTIFY_SLACK_SNIPPET_CHANNEL environment variable and the snippet_channel option in the TOML configuration file have been deprecated. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Test Updates:

• internal/cli/cli_test.go and internal/config/config_test.go: The tests have been updated to reflect the changes in the CLI and configuration files. The channel parameter has been replaced with channel-id and the -filetype option has been replaced with -snippet-type. [1] [2] [3] [4] [5] [6] F93271deL112R99)