Allow custom separators for append and prepend from profile

[SoShiny]

fixes #15908

This is pertains to the [question] issue #15908 about mixed Windows and Unix paths in Msys2 when appending to INCLUDE or similar variables in the profile.
This proposed solution introduces the a notation like "+=(sep=;)" to gain fine grained control over the separator used for appending or prepending, and is intended to serve as a more concrete basis for discussion.

While adding the tests for this, I noticed that there might be an issue with TestProfileEnvRoundTrip::test_prepend. It reverses the order of definitions. Maybe someone can comment on this. It seems wrong, and was previously untested.

As this is for discussion, no documentation was added.

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