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/build-tests/nested_runcontext/folder2/default.project.json

@@ -0,0 +1,7 @@
+{
+  "name": "nested_runcontext",
+  "emitLegacyScripts": true,
+  "tree": {
+    "$path": "folder3"
+  }
+}