Allow passing argument to jsonnet binary

[cemoktra]

Hi,

i'm using jrsonnet binary as jsonnet binary replacement and use an experimental feature there, for that reason i need to pass an extra argument to the binary. It would be nice to allow setting this via the language server to use inside of vs code rather than command line

[julienduchesne]

For most things, we do not use any binary because we do not eval, we only parse the AST (using the go lib)

This could possibly be done for eval diagnostics though