add support for character sequences

[delan]

• named character sequences
  • https://www.unicode.org/reports/tr34/
  • https://www.unicode.org/Public/14.0.0/ucd/NamedSequences.txt
• standardized variation sequences
  • https://www.unicode.org/versions/Unicode14.0.0/ch23.pdf § 23.4
  • https://www.unicode.org/Public/14.0.0/ucd/StandardizedVariants.txt
• emoji variation sequences
  • https://www.unicode.org/reports/tr51/
  • https://www.unicode.org/Public/14.0.0/ucd/emoji/emoji-variation-sequences.txt
• ideographic variation sequences?
  • https://www.unicode.org/reports/tr37/
  • https://www.unicode.org/ivd/data/2020-11-06/IVD_Sequences.txt
• other emoji sequences (UTS #51)
  • https://www.unicode.org/Public/emoji/14.0/emoji-sequences.txt
  • https://www.unicode.org/Public/emoji/14.0/emoji-zwj-sequences.txt
  • https://www.unicode.org/Public/emoji/14.0/emoji-test.txt
• open-ended sequence view for unknown sequences?