[FEAT] Support SDKMAN! Package Count

### Description

Add [SDKMAN!](https://sdkman.io) package count to the output.

### Motivation

I think this falls under the same category as other package managers. SDKMAN! is popular and widely used across multiple Unix platforms to install system-wide SDKs and frameworks.

### Additional context

This should be relatively easy and useful for those using SDKMAN! to install SDKs and packages. Here's how to detect the count:
1. Get the candidates' directory from the variable `$SDKMAN_CANDIDATES_DIR`. If it is empty, then SDKMAN! is not installed.
2. Count the number of **_non-empty_** directories inside `$SDKMAN_CANDIDATES_DIR`. SDKMAN! can leave empty directories for uninstalled SDK candidates, so it is important to count the non-empty directories only (i.e., a bit more involved than a simple `getNumElements` call).

Note that this counts every SDK or framework installed as 1 (even if there are multiple versions installed for it), which is, in my opinion, the correct thing to do.