Add cli for use in NPM scripts

When used in NPM scripts, `setup-micromamba` provides a convenient way to
download micromamba and create conda environments.

Set options through environment variables in invocation. If micromamba has
already been downloaded or an environment already exists, it will not be
recreated.

Example:

```
  "scripts": {
  [...]
    "micromamba": "MICROMAMBA_ROOT_PATH=micromamba CREATE_ENVIRONMENT=true CREATE_ARGS=\"python=3.11\" ENVIRONMENT_NAME=test-env setup-micromamba",
  [...]
```

We re-use the mocking core. Determination of the core backend is moved
to a separate module to prevent duplication and circular imports.

The default micromamba bin path is set to
`${MICROMAMBA_ROOT_PATH}/micromamba` if `MICROMAMBA_ROOT_PATH` is set.

Have the mocking core respect the log level.

README.md

@@ -4,6 +4,9 @@
 
 GitHub Action to set up the [micromamba](https://github.com/mamba-org/mamba#micromamba) package manager.
 
+Also useful to download micromamba and create conda environments in npm
+scripts.
+
 ## Usage
 
 ```yml
@@ -367,6 +370,24 @@ For further information, see
 [`jobs.<job_id>.steps[*].shell`](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstepsshell) 
 and [this thread](https://github.com/orgs/community/discussions/25061).
 
+## Use in NPM scripts
+
+When used in NPM scripts, `setup-micromamba` provides a convenient way to
+download micromamba and create conda environments.
+
+Set options through environment variables in invocation. If micromamba has
+already been downloaded or an environment already exists, it will not be
+recreated.
+
+Example:
+
+```
+  "scripts": {
+  [...]
+    "micromamba": "MICROMAMBA_ROOT_PATH=micromamba CREATE_ENVIRONMENT=true CREATE_ARGS=\"python=3.11\" ENVIRONMENT_NAME=test-env setup-micromamba",
+  [...]
+```
+
 ## Development
 
 1. Clone this repository.