Fab extendTheme capitalization discrepency

[s0rta]

The native base Fab component is capitalized as such, but the theme component is capitalized as FAB, this note is made nowhere in the docs and can be confusing to new users.

I'm down to write a pr, but I don't know which way y'all would want to fix this

1. add a note to the styling area on the fab page
2. add a fab example on the extendTheme page
3. allow extendTheme to work with a non all caps version of Fab