prompt: update powerlevel10k submodule to the latest commit

A dozen bug fixes. Most important:

- rbenv and a few other segments didn't work if IFS was set
  to something unusual.
- vcs segment couldn't properly apply subsegment style if
  color overrides used mnemonic names.
- the check for .p10k.zsh already being sourced was too strict.

modules/prompt/external/powerlevel10k

@@ -1 +1 @@
-Subproject commit 3fe113f91d0f819ad33c90608c3f293a1559324a
+Subproject commit 635b0d7c99a22d996b5ca43efb56e9ec6b1ebdb4