Support bevy 0.13

[jpedrick]

I attempted to use bevy_dsp with bevy 0.13 (from main) and I was unable to get it working due to the removal of TypeUuid and App::add_asset => App::init_asset

I was able to get to the point where DspSource needs to implement GetTypeRegistration

   Compiling bevy_fundsp v0.4.0 (/Users/jpedrick/Development/bevy_fundsp)
error[E0277]: the trait bound `DspSource: GetTypeRegistration` is not satisfied
   --> /Users/jpedrick/Development/bevy_fundsp/src/lib.rs:93:30
    |
93  |             .register_type::<DspSource>()
    |              -------------   ^^^^^^^^^ the trait `GetTypeRegistration` is not implemented for `DspSource`
    |              |
    |              required by a bound introduced by this call
    |
    = help: the following other types implement trait `GetTypeRegistration`:
              bool
              char
              isize
              i8
              i16
              i32
              i64
              i128
            and 389 others
note: required by a bound in `App::register_type`
   --> /Users/jpedrick/Development/bevy/crates/bevy_app/src/app.rs:767:29
    |
767 |     pub fn register_type<T: bevy_reflect::GetTypeRegistration>(&mut self) -> &mut Self {
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::register_type`

For more information about this error, try `rustc --explain E0277`.