add support for variation sequences

[delan]

• 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