[FEATURE] Assign alphabet icons to apps without themed icons

[x9136]

Describe the feature

An option that allows you to optionally assign alphabet icons to apps that do not have themed icons.

https://github.com/LawnchairLauncher/lawnicons/issues/1941

Why would it be useful to add?

Alphabet icons allow you to increase visual consistency. The option will help you quickly install themed icons on apps that lack these icons. It will remain relevant for at least the next 5 years until there are 18-20k icons in Lawnicons (now ~6k).

Did the feature exist in Lawnchair v2? (Play Store version)

No

Example(s)

Additional context

No response