feat: add spellcheck highlighting support

Introduce optional spellcheck highlighting in the input window using
the Enchant-2 library.

```
/spellcheck on
/spellcheck lang en_US
```

New theme color `input.misspelled`.

Fixes: https://github.com/profanity-im/profanity/issues/183
Signed-off-by: Michael Vetter <jubalh@iodoru.org>

tests/unittests/ui/stub_ui.c

@@ -1008,6 +1008,10 @@ cons_splash_setting(void)
 {
 }
 void
+cons_spellcheck_setting(void)
+{
+}
+void
 cons_vercheck_setting(void)
 {
 }