Return charset of ISO's specification DIMENSION, CHARS, and FINAL-CHAR.
ISO 2022's designation sequence (escape sequence) distinguishes charsets
by their DIMENSION, CHARS, and FINAL-CHAR,
whereas Emacs distinguishes them by charset symbol.
See the documentation of the function `charset-info' for the meanings of
DIMENSION, CHARS, and FINAL-CHAR.
(fn DIMENSION CHARS FINAL-CHAR)