Add RunContext support for script outputs (#765)

Resolves #667

This PR:

- Introduces a new field in the project file: `scriptType` which has the
default value of `Class` (in parity with previous versions), but can
also be `RunContext`.
- This is then passed to `InstanceContext` from the `Project` struct.
- This then changes the RunContext in the lua `snapshot_middleware`

---------

Co-authored-by: Micah <dekkonot@rocketmail.com>

src/snapshot_middleware/mod.rs

@@ -38,7 +38,7 @@ use self::{
     util::PathExt,
 };
 
-pub use self::project::snapshot_project_node;
+pub use self::{project::snapshot_project_node, util::emit_legacy_scripts_default};
 
 /// The main entrypoint to the snapshot function. This function can be pointed
 /// at any path and will return something if Rojo knows how to deal with it.