Clarify key to path edge cases with multiple delims

[aajtodd]

When implementing #45 several questions (example) came up around various edge cases w.r.t delimiters

I would expect the following behavior, you should probably add tests

key="many///delims-in-a-row" prefix="" delim="/" -> "many/delims-in-a-row"
key="//delims-at-start" prefix="" delim="/" -> "delims-at-start"
key="prefix///then-delims" prefix="prefix" delim="/" -> "then-delims"
key="prefix///then-delims" prefix="prefix/" delim="/" -> "then-delims"

NOTE: All but the first one here fail in the Java v2 TM today

what if stripped contains multiple delimiters in a row? e.g. key="a///1.txt" prefix="a/"
does path_clean handle that?

off the top of my head, if there are multiple delimiters, they should be collapsed to 1. Same way that typing
cd my-rust-project////////target gets the same result as typing cd my-rust-project/target

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We need to clarify these edge cases and drive alignment and update the SEP if necessary. Many of them don't work the way we thought they should and actually result in errors in the current Java v2 TM. We don't want to create incompatibilities between TM implementations and make valid input for one TM suddenly invalid for another and vice versa.

There is also a test case in the SEP that is marked as being a success but actually fails currently in both Rust and Java TM:

            // FIXME - figure out if this test case is valid, Java v2 TM fails with exception stating it's outside the target directory
            // success_path_test("2023/Jan-1.png",	Some("2023"),	Some("-"),	"test/Jan/1.png"),