feat(go): Create go_prompt_info to add Go version to theme prompts (#8924)

* Create go_prompt_info to add go version to prompts

* golang: move go_prompt_info to plugin, guard and quote output

Addresses review feedback: the prompt helper only belongs to Go
developers, so it moves out of lib/ (sourced for every shell) into
plugins/golang/golang.plugin.zsh. Adds a $+commands[go] guard so
users without Go installed don't spawn a process or see a stderr
error on every prompt render, quotes the version output the same
way the rvm/nvm helpers do so a % can't be read as a prompt escape,
and adds the dummy go_prompt_info stub to
lib/prompt_info_functions.zsh for themes that call it when the
plugin isn't loaded.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0193G8TyNqmNnv6v9HsVafGa

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

lib/prompt_info_functions.zsh

@@ -21,6 +21,7 @@ function chruby_prompt_info \
   azure_prompt_info \
   tf_prompt_info \
   conda_prompt_info \
+  go_prompt_info \
 {
   return 1
 }