feat(corfu): add snippets

Yasnippet is now properly integrated! A previosly-unset default has now
been given to `corfu-on-exact-match`. With snippets, it causes immediate
expansion upon single match by default, so we set it to nil and
recommend against changing it in the README.

modules/completion/corfu/README.org

@@ -33,6 +33,7 @@ utilities for fine-tuning. Only some of common behaviors are documented here.
 - [[doom-package:nerd-icons-corfu]] if [[doom-module::completion corfu +icons]]
 - [[doom-package:orderless]] if [[doom-module::completion corfu +orderless]]
 - [[doom-package:corfu-terminal]] if [[doom-module::os tty]]
+- [[doom-package:yasnippet-capf]] if [[doom-module::editor snippets]]
 
 ** Hacks
 /No hacks documented for this module./
@@ -50,7 +51,7 @@ languages may lack code completion support altogether). Run ~$ doom doctor~ to
 find out if you're missing any dependencies. Note that Corfu may have support
 for completions in languages that have no development intelligence, since it
 supports generic, context insensitive candidates such as file names or recurring
-words.
+words. Snippets may also appear in the candidate list if available.
 
 * TODO Usage
 #+begin_quote