[FEATURE]: Add shfmt formatter for shell scripts

[anntnzrb]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Add built-in support for shfmt as a formatter for shell scripts.

Why is this needed?

• OpenCode has bash-language-server for LSP but no shell formatter
• shfmt is the de facto standard shell formatter (like gofmt for Go)
• Follows existing formatter patterns

Proposed implementation

• Extensions: .sh, .bash (NOT .zsh - shfmt doesn't support Zsh)
• Command: shfmt -w $FILE
• Detection: Binary check via Bun.which("shfmt")

[anntnzrb]

PR: #6204