feat: add an update command for self updating

[mcncl]

Description

Adds a command so that users can run bk update to update to the latest version, when the binary is used, or updates message for updating via brew or mise

Changes

• introduces an update command for use via bk update
  • outputs message for updatign when brew or mise used instead
• adds an internal update package which determines if self updating can be used
• adds testing

Testing

• Tests have run locally (with go test ./...)
• Code is formatted (with go fmt ./...)

Disclosures / Credits

Used Codex locally to review the changes and provide feedback on use of idiomatic Go.