Option to switch back to old Operator parsing

[domluna]

v1 parses all operators as Identifier now. JuliaFormatter relied on JuliaSyntax.is_operator(node) since it treats operators different than identifiers, mainly for spacing around calls.

I'm wondering there is either:

1. A way to keep the old behaviour via an argument to JuliaSyntax.parseall(...)
2. Some function that I can use such as JuliaSyntax.is_operator(node) (currently doesn't work) to verify it's an operator even though they are now parsed as Identifier.

[pfitzseb]

Ref #545 (comment) maybe? Adding a convenience function does seem like a good idea though.