fix(parser): don't create enums for union types

[lars-reimann]

Closes #966.

Summary of Changes

If we detect that a parameter has a union type, we no longer create an @enum annotation for it.

[github-actions]

MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ CREDENTIALS	secretlint	yes		no	3.34s
✅ GIT	git_diff	yes		no	0.01s
✅ JSON	eslint-plugin-jsonc	2	0	0	3.22s
✅ JSON	jsonlint	2		0	1.0s
✅ JSON	prettier	2	0	0	1.99s
✅ JSON	v8r	2		0	1.93s
✅ PYTHON	bandit	1		0	0.27s
✅ PYTHON	black	1	0	0	0.32s
✅ PYTHON	flake8	1		0	0.33s
✅ PYTHON	isort	1	0	0	0.13s
✅ PYTHON	mypy	1		0	7.76s
✅ PYTHON	pylint	1		0	1.66s

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[github-actions]

🎉 This PR is included in version 1.57.1 🎉

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