Allow multiple plugins to be specified

[wbobeirne]

PLUGIN_DIR only accepts a single directory, so if we were to have multiple plugins, we could only choose to use 1 at a time. We could either just split on comma, have the variable accept a JSON array, or change it to a flag that can be specified multiple times (e.g. --plugin /path/to/plugin1 --plugin /path/to/plugin2)

[code-asher]

fwiw PLUGIN_DIR was actually just for development, the original plan was that plugins would be in the plugins directory inside the code-server directory. So we could have a flag that sets that directory, or extract plugins to that directory when downloading everything without setting any variables.

[code-asher]

It should def be made configurable, not just hardcoded to plugins like it currently is.