Add centaur-tabs-local-mode-if-enabled

[DivineDominion]

I am still experimenting with centaur-tabs-mode and found that the centair-tabs-local-mode hook from the README was a bit too invasive: when I turn (centaur-tabs-mode -1) off, then the local minor mode would error out. That resulted in my org agenda not displaying at all :)

My suggestion would be to add one of the following:

1. to offer a function in the library that doesn't error out (this PR)
2. to offer this function in the README example instead of the main lib
3. to change the minor mode and just do nothing instead of erroring-out (would be my favorite to be honest, but not sure if this is intentional)

@ema2159 @nebhrajani-a What would you think is the best approach? Happy to amend the PR accordingly

[ema2159]

Hi! I very much appreciate your PR, however, is it really necessary to add a new function? Wouldn't it be easier to just write the following for your hooks?

(lambda () ((when centaur-tabs-mode
    (centaur-tabs-local-mode)))

[DivineDominion]

@ema2159 I'm now more leaning towards making the minor-mode not error out instead of adding this band-aid. Do you have an opinion on that?