Allow escaping key seperators : and /

[pi0]

Related: #304

Unstorage reserves key separators such as : and /. We might find a way to allow escaping. Double :: and // might be one idea (although is a potential breaking change)

[pwFoo]

Would be gread to have configurable delimiter to be more compatible to different type of keys? Looks like : and / are hardcoded path delimiter with unexpected behaviour if both delimiters in key.
I know any people use / as path delimiter, but it could be conflict with special keys like ``SubtleCrypto.exportKey` based strings.

Opened issue #568 because of my key delimiter problem.