Fix scalatra on Windows

[martin-mfg]

Using the scalatra generator on Windows crashes OpenAPI Generator with java.lang.IllegalArgumentException: character to be escaped is missing. This PR fixes that.

File.separator returns a single backslash on Windows, which is not a valid value for replaceAll.

To guard against similar problems, I could add a unit test that simply invokes each existing generator on a small input spec. Just to ensure the generator runs without crashes. This would then automatically be checked by the windows workflow (and others as well).
Let me know if that would be helpful.

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[wing328]

Let me know if that would be helpful.

yup that would be helpful to cover the issue on Windows moving forward.