seq: additional feature: ranges of letters

Feature description

seq supports ranges of letters in addition to numbers

e.g.:

$ seq -s ' ' b
a b
$ seq -s ' ' b e
b c d d 
$ seq -s ' '  F 2 L
F H J L

-s and -w will not be supported with letter ranges for obvious reasons.
Increment still has to be a number.
If the start argument is a letter, the end argument also has to be a letter of the same case.

Am I willing to implement this?

Yes

Additional Context

Fish shell doesn't have ranges with [] like bash -c 'echo [a-z]* [1-3]*' and instead suggests to use seq, however seq doesn't support letters.

If you don't want this in uutils, I may create a separate seq tool that supports this.

Start by locating the seq implementation and its existing numeric range handling. Define the letter-range cases described in the issue, including case matching, numeric increments, and the stated -s and -w restrictions; done means these examples and boundary cases behave consistently.