[linux] Define an alias in fish shell

I found the prior answers and comments to be needlessly incomplete and/or confusing. The minimum that I needed to do was:

  1. Create ~/.config/fish/config.fish. This file can optionally be a softlink.
  2. Add to it the line alias myalias echo foo bar.
  3. Restart fish. To confirm the definition, try type myalias. Try the alias.