Implement errexit option

[magicant]

• Implement Env::apply_errexit
• Push Frame::Condition in if/while/until conditions
• Push Frame::Condition in negated command
• Push Frame::Condition in and-or lists
• Exit on simple command returning non-zero exit status
• Exit on subshell command returning non-zero exit status
• Exit on function definition command returning non-zero exit status
• Exit on redirection error in compound command

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• Exit on expansion error in for loop word → not needed because expansion error exits the shell by default implemented in another PR
• Exit on assignment error in for loop variable → not needed because assignment error exits the shell by default implemented in another PR
• Exit on expansion error in case command word or pattern → not needed because expansion error exits the shell by default implemented in another PR
• Exit on double bracket command returning non-zero exit status → not implemented in this PR because the double bracket command itself is not yet implemented