Improve handling of `modifiers`

In many cases to move towards a more ideal AST (and align with babylon), we want to apply modifiers as booleans on the result node, rather than an arbitrary `modifiers` array of converted nodes.

I will work on a generic solution for this as part of a bug fix to ensure that `const enum` does not lose the `const` part in the AST. This bug was introduced by the enum refactor.