Map ksh files to bash syntax

[jwbowen]

If there are no objections to the idea, I'd like to add ksh to the syntax
mapping for bash highlighting. I was going to do it myself and just send
a PR, but it wasn't readily apparent what the best way to add it would be.

I'd still be happy to add it with a bit of guidance on where to make changes.

[keith-hall]

For those that have never heard of ksh before, https://en.wikipedia.org/wiki/KornShell says:

KornShell is backward-compatible with the Bourne shell

So no objection here to the mapping. Probably the easiest way is to take inspiration from a previous PR adding a mapping:
https://github.com/sharkdp/bat/pull/2615/files