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

* Add spackenv plugin to handle Spack environment

* Use ":t" instead of 'basename'

plugins/spackenv/README.md

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+# spackenv
+
+Based on the virtualenv plugin.
+
+The plugin displays information of the created Spack environment and allows background theming.
+
+To use it, add `spackenv` to the plugins array of your zshrc file:
+```
+plugins=(... spackenv)
+```
+
+The plugin creates a `spackenv_prompt_info` function that you can use in your theme, which displays
+the basename of the current `$SPACK_ENV`. It uses two variables to control how that is shown:
+
+- `ZSH_THEME_SPACKENV_PREFIX`: sets the prefix of the SPACK_ENV. Defaults to `[`.
+
+- `ZSH_THEME_SPACKENV_SUFFIX`: sets the suffix of the SPACK_ENV. Defaults to `]`.

plugins/spackenv/spackenv.plugin.zsh

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+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=)}"
+}