v0.0.0-9850-ga0ecdeb68

Add parsing support for spawns of impl-style procs.

Spawns of legacy procs will continue to look like:
```spawn Foo(config_args);```

Spawns of impl-style procs will look like an invocation of a trait-derived function:
```foo_instance.spawn();``` or ```Foo::new(args).spawn()``` or similar. This will become a regular Invocation node and not a Spawn node. We will not need Spawn nodes anymore once we get rid of the legacy syntax.

To make this work, this change makes "spawn" not a keyword anymore.

PiperOrigin-RevId: 899782677