Add break and continue loop control commands

[wolfv]

Implement break and continue builtin commands to provide loop control flow:

• Add Break and LoopContinue variants to ExecuteResult enum to signal loop control flow separately from normal execution and exit
• Create break and continue builtin commands that return the appropriate ExecuteResult variants
• Update for, while, and until loop execution to handle Break (exit loop) and LoopContinue (skip to next iteration) results
• Propagate loop control flow through all command execution paths (subshells, if statements, case statements, pipelines, etc.)
• Add comprehensive tests for break and continue in for loops, including nested loops

All existing tests pass. The implementation follows bash semantics where:

• break exits the innermost enclosing loop
• continue skips to the next iteration of the innermost enclosing loop
• Both commands outside of loops behave as no-ops (return 0)