Problem:
`getchar.c` is a quirky name and doesn't align with our existing
`os/input.c` and `tui/input.c` modules, which encompass the same topic
at different layers.
Solution:
Rename `getchar.c` => `input.c` for discoverability.
Introduce `dialog.c` for functionality related to showing confirm
prompts, dialogs, etc.
These are not needed after #35129 but making uncrustify still play nice
with them was a bit tricky.
Unfortunately `uncrustify --update-config-with-doc` breaks strings
with backslashes. This issue has been reported upstream,
and in the meanwhile auto-update on every single run has been disabled.
