emsdk

Improve ordering in `emsdk list` (#1180)

We have an existing `version_key` helper function for sorting versions.

It also does a better job, producing output like:

```
All recent (non-legacy) installable versions are:
         3.1.31
         3.1.31-asserts
         3.1.30
         3.1.30-asserts
         3.1.29
         3.1.29-asserts
```

Rather than:

```
All recent (non-legacy) installable versions are:
         3.1.31
         3.1.30
         3.1.29
         3.1.28
         3.1.27
```

(with -assert versions listed after 3.1.0)