`ParseResult#continuable?` for code ending with whitespace or newline

[tompng]

I expect these to be all true.

Prism.parse("class A").continuable? #=> true
Prism.parse("class A\n").continuable? #=> false
Prism.parse("def f").continuable? #=> true
Prism.parse("def f ").continuable? #=> false
Prism.parse("def f\n").continuable? #=> false
Prism.parse("def f\n  ").continuable? #=> true
Prism.parse("(").continuable? #=> true
Prism.parse("( ").continuable? #=> false
Prism.parse("(\n").continuable? #=> false
Prism.parse("1 +").continuable? #=> true
Prism.parse("1 +\n").continuable? #=> false

I found these with IRB's test code.