Lexical: Added testing for some added shortcuts

Also:
- Added svg loading support (dummy stub) for jest.
- Updated headless test case due to node changes.
- Split out editor change detected to where appropriate.
- Added functions to help with testing, like mocking our context.

dev/build/svg-blank-transform.js

@@ -0,0 +1,14 @@
+// This is a basic transformer stub to help jest handle SVG files.
+// Essentially blanks them since we don't really need to involve them
+// in our tests (yet).
+module.exports = {
+    process() {
+        return {
+            code: 'module.exports = \'\';',
+        };
+    },
+    getCacheKey() {
+        // The output is always the same.
+        return 'svgTransform';
+    },
+};