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.

jest.config.ts

@@ -185,6 +185,7 @@ const config: Config = {
   // A map from regular expressions to paths to transformers
   transform: {
     "^.+.tsx?$": ["ts-jest",{}],
+    "^.+.svg$": ["<rootDir>/dev/build/svg-blank-transform.js",{}],
   },
 
   // An array of regexp pattern strings that are matched against all source file paths, matched files will skip transformation