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

* Add spackenv plugin to handle Spack environment

* Use ":t" instead of 'basename'

plugins/spackenv/README.md

@@ -0,0 +1,17 @@
+# 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 `]`.