use new api for uploading snippet

[catatsuy]

refs: #148

This pull request primarily focuses on changes related to the Slack API, specifically the transition from using channels to using channel IDs for file uploads. The changes also include the deprecation of the snippet_channel option and the addition of the channel_id option in the configuration. There are also updates to the README.md file to reflect these changes, and the addition of the github.com/google/go-cmp dependency in the go.mod file.

Changes in the Slack API:

• README.md: The documentation was updated to reflect the changes in the Slack API, including the transition from channels to channel IDs for file uploads, and the deprecation of the snippet_channel option. [1] [2] [3]
• internal/cli/cli.go: The code was updated to use channel IDs instead of channels for file uploads. This includes changes in the Run and uploadSnippet functions. [1] [2] [3] [4] [5]
• internal/cli/cli_test.go: The tests were updated to reflect the changes in the Slack API. This includes changes in the TestUploadSnippet function. [1] [2] [3] [4]
• internal/config/config.go: The configuration was updated to use channel_id instead of snippet_channel. This includes changes in the Config struct, the LoadEnv and LoadTOML functions, and the slackConfig struct. [1] [2] [3] [4]
• internal/config/config_test.go: The tests were updated to reflect the changes in the configuration. This includes changes in the TestLoadTOML and TestLoadEnv functions. [1] [2] [3] [4] [5] [6]
• internal/config/testdata/config.toml: The configuration file was updated to use channel_id instead of snippet_channel.

Changes in dependencies:

• go.mod: The github.com/google/go-cmp dependency was added.