Implement support for turning .json files into Lua modules (#308)

* Stub implementation

* Flesh out feature and add tests. Other snapshots currently failing.

* Blacklist .meta.json in JSON handler

* Write to correct property (Source) instead of Value

* Update changelog

rojo-test/build-test-snapshots/rojo_test__build_test__json_as_lua.snap

@@ -0,0 +1,23 @@
+---
+source: rojo-test/src/build_test.rs
+expression: contents
+---
+<roblox version="4">
+  <Item class="ModuleScript" referent="0">
+    <Properties>
+      <string name="Name">json_as_lua</string>
+      <string name="Source">return {
+	["1invalidident"] = "nice",
+	array = {1, 2, 3},
+	["false"] = false,
+	float = 1234.5452,
+	int = 1234,
+	null = nil,
+	object = {
+		hello = "world",
+	},
+	["true"] = true,
+}</string>
+    </Properties>
+  </Item>
+</roblox>