Add --create to spack env activate (#40896)

Add `--create` option to `env activate` to allow users to create and activate in one command.


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Co-authored-by: Wouter Deconinck <wdconinc@gmail.com>
Co-authored-by: Tamara Dahlgren <35777542+tldahlgren@users.noreply.github.com>
Co-authored-by: psakievich <psakievich@users.noreply.github.com>

lib/spack/docs/environments.rst

@@ -142,6 +142,21 @@ user's prompt to begin with the environment name in brackets.
    $ spack env activate -p myenv
    [myenv] $ ...
 
+The ``activate`` command can also be used to create a new environment, if it is
+not already defined, by adding the ``--create`` flag. Managed and anonymous
+environments, anonymous environments are explained in the next section,
+can both be created using the same flags that `spack env create` accepts.
+If an environment already exists then spack will simply activate it and ignore the
+create specific flags.
+
+.. code-block:: console
+   
+   $ spack env activate --create -p myenv
+   # ...
+   # [creates if myenv does not exist yet]
+   # ...
+   [myenv] $ ...
+
 To deactivate an environment, use the command:
 
 .. code-block:: console