Replace comma-separated envvars with comma+whitespace separated envvars

### Enhancement Type

Improve an existing feature

### Describe the enhancement

Seems like there are some envvars that split on commas, some on commas+newlines, some on commas+whitespace, and others on just whitespace. I propose replacing all of the first type with comma+whitespace splits unless they rely on newlines being retained (which I don't see being an issue). In places where commas are required (e.g. in [mc-image-helper](https://github.com/itzg/mc-image-helper#mcopy), and are they really required?), values can first be split on commas+whitespace, then re-joined on commas:
```sh
IFS=,$IFS # prepend comma to $IFS
plugins=($PLUGINS) # split on $IFS (comma+whitespace)
mc-image-helper mcopy ... "$*" # join with comma
IFS=${IFS#?} # restore $IFS
```

The motivation behind this issue is this snippet from my docker-compose.yml:
```yaml
SPIGET_RESOURCES:
# spark,discordsrv,deathchest,pluginmanager,spark,veinminer #,vault,essentialsx
57242,18494,101066,69061,57242,12038 #,34315,9089
```
Notice how I accidentally duplicated one of the plugins. Ignoring how cryptic YAML multiline indicators are, I would prefer to write this instead:
```yaml
SPIGET_RESOURCES: >-
57242 # spark
18494 # discordsrv
101066 # deathchest
69061 # pluginmanager
12038 # veinminer
#34315 # vault
#9089 # essentialsx
```

I can work on a PR for this if we're in agreement!

The issue names no files or tests. Start by searching the repository for environment-variable parsing and comma-splitting, then inventory which variables use each delimiter style. Compare the existing behavior with the SPIGET_RESOURCES example and mc-image-helper case; done means compatible existing configurations while supporting comma-plus-whitespace-separated values.

Written by the indexing model from the issue text.