feat(spackenv): Adding Spackenv plugin (#13191)

* Add spackenv plugin to handle Spack environment

* Use ":t" instead of 'basename'

plugins/spackenv/spackenv.plugin.zsh

@@ -0,0 +1,5 @@
+function spackenv_prompt_info(){
+  [[ -n ${SPACK_ENV} ]] || return
+  export SPACK_ENV_PROMPT=${SPACK_ENV:t}
+  echo "${ZSH_THEME_SPACKENV_PREFIX=(}${SPACK_ENV:t:gs/%/%%}${ZSH_THEME_SPACKENV_SUFFIX=)}"
+}