Add Micromamba support (nushell#800)

I initially tried to get Micromamba to work with Nushell by using the
script from [this
issue](nushell#578). But even after
fixing the syntax errors I couldn't activate any env.
With these modifications, Micromamba seems to be working fine.  
Please note that I'm far from a Nu expert so my code might not be very
idiomatic. If that is the case please tell me so and I'll be happy to do
the necessary changes.

modules/virtual_environments/nu_conda_2/conda.nu

@@ -2,7 +2,13 @@ def --env load-conda-info-env [] {
  if (not (has-env CONDA_INFO)) {
  export-env {
  $env.CONDA_INFO = (
- if not (which mamba | is-empty) {
+ if not (which micromamba | is-empty) {
+ mut info = micromamba env list --json | from json
+ let extra_info = micromamba info --json | from json
+ $info.envs_dirs = $extra_info."envs directories"
+ $info.root_prefix = $extra_info."base environment"
+ $info
+ } else if not (which mamba | is-empty) {
  (mamba info --envs --json --no-banner | from json)
  } else if not (which conda | is-empty) {
  (conda info --envs --json | from json)
@@ -21,7 +27,7 @@ export def --env activate [
  load-conda-info-env
  let conda_info = $env.CONDA_INFO
  if ($conda_info == null) {
- print "Error: No Conda or Mamba install could be found in the environment. Please install either and add them to the environment. See: https://www.nushell.sh/book/environment.html for more info"
+ print "Error: No Conda, Mamba or Micromamba install could be found in the environment. Please install either and add them to the environment. See: https://www.nushell.sh/book/environment.html for more info"
  return
  }
 
@@ -173,4 +179,4 @@ def system-path [] {
 
 def has-env [name: string] {
  $name in $env
-}
+}