write multiple Bidi_Class @missing lines

[markusicu]

For UTC action item 168-A7: Propose a change in one or more UCD files to use multiple @missing lines; for Unicode version 15.0. See document L2/21-126 item D2.

• refactor and modernize Bidi_Class defaults into new class DefaultValues
  • also try to fix some historical Bidi_Class default-value assignments back to Unicode 3.0
• in DerivedBidiClass.txt: write multiple Bidi_Class @missing lines
  • one per default-value range where bc does not default to L
  • except no @missing lines for bc=BN
• omit explicit code point / range lines for unassigned code points with default values
  • the only gc=Cn with explicit data lines are now those with bc=BN
• adjust the org.unicode.props.PropertyParsingInfo parser to handle multiple @missing lines