Option to use posix exit code upon fatal signal

[73rhodes]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions.

Description of the Change

This PR introduces a --posix-exit-codes boolean command-line option. If this option is specified, a fatal signal (eg. SIGABRT, et al) will cause the process to exit with a standard posix exit code (128 + the numeric ID of the signal). Making this optional supports existing consumers that depend on the original exit codes, while solving issues for toolchains that expect standard posix behavior. Specifically, this option addresses a problem where out-of-memory crashes go undetected because the mocha exit codes are non-standard.

Alternate Designs

The alternatives considered are not in the scope of the mocha project.

Why should this be in core?

This option is implemented in the core repository where signal handling occurs.

Benefits

This PR provides a solution for mochajs#3559 and various downstream issues reported by mocha consumers; it preserves non-zero exit codes when mocha is spawned as a child process via various CI/CD or reporting tools and prevents silently swallowing out-of-memory errors.

Possible Drawbacks

Introduces another option; requires docs; permits known non-standard exit code behavior to remain as the default.

Applicable issues

mochajs#3559
mochajs#3893
mochajs#2445
mochajs#2438
istanbuljs/nyc#798
cypress-io/cypress#24695

• No breaking changes; this is an ehancement (minor release)