Use const instead of immutable for compatibility with new libdparse

[Hackerpilot]

Looks like the libdparse token changes weren't as backwards-compatible as we hoped. See #799 (comment)

[WebFreak001]

hm I'm not quite sure how these changes are related to the token structure changes. Did including the const arrays make it impossible to be immutable?

In this case would changing

    const(typeof(this))[] leadingTrivia;
    /// ditto
    const(typeof(this))[] trailingTrivia;

in libdparse to

    immutable(typeof(this))[] leadingTrivia;
    /// ditto
    immutable(typeof(this))[] trailingTrivia;

be the needed fix? The appender data is dup'd anyway, it could probably be changed to idup without issues.

[WebFreak001]

fixing it here isn't the right approach imo, I submitted a PR to libdparse to fix it there instead