browser-plugin: Rework scriptable method argument parsing and dispatch

Since we're going to move to a much more complicated (async!) solution
in a little bit, we're going to require a lot more machinery to handle
that. To help with that, let's rework invocation dispatch so that it's
more generic. Introduce a parse_args system similar to gjs_parse_args,
use X Macros to help with the repetitive parts of the method dispatch.
This shouldn't cause any API breaks, so API_VERSION should still be 4.

https://bugzilla.gnome.org/show_bug.cgi?id=679099