Move each builtin command to its own package

[AlexandreYang]

What does this PR do?

Refactors the interp/builtins package so each builtin command lives in its own sub-package under interp/builtins/<command>/. The root builtins package retains only the shared types (HandlerFunc, CallContext, Result) and registry (Register, Lookup).

Motivation

Isolating each builtin into its own package improves modularity: each command has a clear boundary, its own tests, and independent dependencies. It also makes the import allowlist test more maintainable as new builtins are added.

Testing

• go build ./interp/... — compiles clean
• go test ./interp/... — all pass (except pre-existing TestHeadPermissionDenied which fails only when running as root)
• go test ./tests/ — all pass including the updated TestBuiltinImportAllowlist
• goimports / gofmt — no formatting changes needed

Checklist

• Tests added/updated
• Documentation updated (if applicable)

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Summary: Each builtin (cat, echo, exit, head, break, continue, true, false) is now a separate Go package that self-registers via init() + builtins.Register(). Shared logic for break/continue lives in builtins/internal/loopctl. The runner wires everything together with blank imports.

Changes:

• Exported Register in interp/builtins/builtins.go
• Created per-builtin packages: cat/, echo/, exit/, head/, break/, continue/, true/, false/
• Created internal/loopctl/ for shared break/continue loop-control logic
• Moved head tests from builtins/tests/head/ into builtins/head/
• Added blank imports in interp/runner_exec.go to trigger init() registration
• Updated TestBuiltinImportAllowlist to walk sub-packages recursively, exempting the parent builtins package and internal/ imports
• Deleted old flat files (cat.go, echo.go, exit.go, head.go, break_continue.go, true_false.go) and tests/ directory

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