maint: honor @missing in DerivedBidiClass and report non values (#201)

Starting with Unicode 15, the provided DerivedBidiClass data file
reports different default values to use for unassigned characters
in different groups.

Process the additional hints for that specific file, and allow
overriding the values later if more specific.

Since that was previously forbidden, change get_other_case() to
report when no valid value could be provided and allow skipping
conflicting rule lines that required that restriction.

While at it, Allow using the long identifiers in `ucptest` with
the `find bidi` command (underscores also allowed).