__fish_print_help should respect $MANPAGER

[ian-h-chamberlain]

It seems that builtins' --help argument seems to display differently than usual man commands:

# Seems to just use my default $PAGER instead of $MANPAGER
MANPAGER='sh -c "col -bx | bat -plman"' printf --help

vs

# Does what I expect
MANPAGER='sh -c "col -bx | bat -plman"' man printf

Since the builtins use the same man-style output it would make sense to use $MANPAGER first, perhaps falling back to $PAGER if unset.

Originally posted by @ian-h-chamberlain in #2170 (comment)

[faho]

Note: printf is a bad example because that's actually not supposed to take any options, and we've fixed that in 3.6.0 - #9132.

(ideally we'd do what we do for e.g. begin --help and print the help if that's literally the command, but that's a bigger thing)