Make hide-mode-line package optional

### Describe your request

I'd like Doom to make `hide-mode-line` an optional module/package, e.g. by making it a flag of module `:ui doom`.
I understand the rationale behind this package, but many users (myself included) are willing to give up a little screen estate of the mode-line for consistent UI (i.e. every displayed buffer has its mode-line visible).

### Briefly explain its use-case

Ideally, users can enable `hide-mode-line` by enabling the `:ui (doom +hide-mode-line)` module.
Currently, disabling `hide-mode-line` is very cumbersome without the use of `advice-add` and similar functions.