Add check to ensure plugin version matches cargo version (#794)

This modifies Rojo's build script to throw a fit if we're building a
plugin with a semver incompatible version. In the process, it moves the
version of the plugin to a file named `Version.txt` that's parsed at
runtime. This should be minimally invasive but it's technically worse
for performance than the hardcoded table and string we had before.

This feels better than a CI check or just manually verifying because it
makes it physically impossible for us to forget since Rojo won't build
with it being wrong.

plugin/default.project.json

@@ -1,25 +1,27 @@
 {
-	"name": "Rojo",
-	"tree": {
-		"$className": "Folder",
-		"Plugin": {
-			"$path": "src"
-		},
-		"Packages": {
-			"$path": "Packages",
-
-			"Log": {
-				"$path": "log"
-			},
-			"Http": {
-				"$path": "http"
-			},
-			"Fmt": {
-				"$path": "fmt"
-			},
-			"RbxDom": {
-				"$path": "rbx_dom_lua"
-			}
-		}
-	}
-}
+  "name": "Rojo",
+  "tree": {
+    "$className": "Folder",
+    "Plugin": {
+      "$path": "src"
+    },
+    "Packages": {
+      "$path": "Packages",
+      "Log": {
+        "$path": "log"
+      },
+      "Http": {
+        "$path": "http"
+      },
+      "Fmt": {
+        "$path": "fmt"
+      },
+      "RbxDom": {
+        "$path": "rbx_dom_lua"
+      }
+    },
+    "Version": {
+      "$path": "Version.txt"
+    }
+  }
+}