Proper way to specify the sys_image in additional args

[MariusDrulea]

Current:

"julia.additionalArgs": [
        "-JC:\\temp\\sys_custom.so",
    ],

Shall be:

"julia.additionalArgs": [
        "-J", "C:\\temp\\sys_custom.so",
    ],

Bug?:
@pfitzseb the config bellow does not work, but I think it's related to vscode itself, not to the julia extension. Notice the space between J and the path to the sys image.

"julia.additionalArgs": [
        "-J C:\\temp\\sys_custom.so",
    ],

PR here: #77