Handle parser errors without crashing

[PNHD]

Fixes #40. Related to #253.

Parser exceptions currently escape from the stdout/stderr stream data callbacks, so invalid parsed output can crash the Node process instead of reaching .end() / PythonShell.run().

This keeps parser failures on the library's existing completion/error surface while preserving EventEmitter semantics:

• parse stdout/stderr chunks before calling emit(), so exceptions thrown by user message / stderr listeners still propagate normally instead of being misclassified as parser failures
• store the first stdout/stderr parser exception and route it through the existing PythonShellError completion path
• prefer a real non-zero Python process error/traceback when both a parser failure and process failure occur, rather than masking the Python error
• document that parser failures can surface through pythonError, .end(), and PythonShell.run()

Regression coverage includes invalid JSON, custom stderr-parser failures, listener exceptions, and parser-vs-process error precedence.

Relationship to #326

#326 addresses the same JSON parse-failure family by adding a public parseError event/class. This PR keeps the existing completion/error surface instead, also handles custom stderr-parser failures, and keeps EventEmitter listener exceptions outside the parser catch path. I am happy to defer to the maintainers' preferred API direction.

Validation on the current branch:

• focused parser/listener/error-precedence suite: 5 passing
• full npm test: 48 passing
• npx prettier --check index.ts test/test-python-shell.ts README.md
• git diff --check