This used the naive `__fish_seen_subcommand_from`, which isn't
powerful enough once you allow for `conda create` and `conda env
create`.
Hattip to jvanheugten for the env completions.
Fixes#9452
* Completion for conda, the package manager
* Make the list of platforms a private variable
* Add commands activate and deactivate
* Avoid clobbering a user-defined function __
* Use Use __fish_seen_subcommand_from to identify subcommand
And treat the case of the first argument as a special case
with function __fish_conda_fist_arg
* Factor out create from loop for option --name
* Fix typo (missing parenthesis in description)
* Start from a blank state by removing completions from conda configuration script
