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>

rojo-test/serve-test-snapshots/end_to_end__tests__serve__scripts_all-2.snap

@@ -24,6 +24,8 @@ instances:
     Name: bar
     Parent: id-2
     Properties:
+      RunContext:
+        Enum: 0
       Source:
         String: "-- Hello, from bar!"
   id-4:

rojo-test/serve-test-snapshots/end_to_end__tests__serve__scripts_all.snap

@@ -24,6 +24,8 @@ instances:
     Name: bar
     Parent: id-2
     Properties:
+      RunContext:
+        Enum: 0
       Source:
         String: "-- Hello, from bar!"
   id-4: