Add fish shell support (#9279)

* share/spack/setup-env.fish file to setup environment in fish shell

* setup-env.fish testing script

* Update share/spack/setup-env.fish

Co-Authored-By: Elsa Gonsiorowski, PhD <gonsie@me.com>

* Update share/spack/qa/setup-env-test.fish

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* updates completions using `spack commands --update-completion`

* added stderr-nocaret warning

* added fish shell tests to CI system


Co-authored-by: becker33 <becker33@llnl.gov>
Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>
Co-authored-by: Elsa Gonsiorowski, PhD <gonsie@me.com>