Test and document builtins.baseNameOf, improve internal baseNameOf()

[roberth]

Motivation

baseNameOf is weird. Deal with it.

Context

Found while at code related to #10303

Priorities and Process

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[Ericson2314]

FWIW programming language standard libraries tend not to remove any slashes:

• https://hackage.haskell.org/package/filepath-1.5.2.0/docs/System-FilePath.html#v:takeBaseName
• https://docs.python.org/3/library/os.path.html
• https://en.cppreference.com/w/cpp/filesystem/path/filename

Perhaps we will eventually (not now!) migrate our internal baseName to work that way too, e.g. as part of #9205. But totally that agreed removing all trailing slashes is more consistent that removing just one.