Add support for 256-color and 24bit ANSI colors in ansi-color

Add support for 256-color and 24bit ANSI colors in ansi-color

* lisp/ansi-color.el (ansi-color--code-as-hex): New function to
convert from 256-color and 24-bit ANSI codes.
(ansi-color--face-vec-face): Add support for ANSI color codes greater
than 16
(ansi-color--update-face-vec): Add support for ANSI codes 38 and 48
which can specify 256-color and 24bit ANSI colors.

* test/lisp/ansi-color-tests.el (ansi-color-tests--strings): Add tests
for ANSI codes 38 and 34