Implement Save without re-prompt and Save as (#1709)

* Implement Save without re-prompt and Save as
Fixes #1668

* Add save-as icon

* Make .excalidraw the default extension

* Only show save as button on supporting browsers

package-lock.json

@@ -3306,9 +3306,9 @@
       "integrity": "sha1-EsJe/kCkXjwyPrhnWgoM5XsiNx8="
     },
     "browser-nativefs": {
-      "version": "0.8.1",
-      "resolved": "https://registry.npmjs.org/browser-nativefs/-/browser-nativefs-0.8.1.tgz",
-      "integrity": "sha512-5XQTR6eg+/hDBVoOKbCnCqUzhD7IP5RG6jCe+J+EaTHo8EnDxjEj3mod3BiEBc/4NfTLEMbrMzUPPY64KwnmNw=="
+      "version": "0.8.2",
+      "resolved": "https://registry.npmjs.org/browser-nativefs/-/browser-nativefs-0.8.2.tgz",
+      "integrity": "sha512-x1dYA6lkpaLZcvvbQ1+/SSDR9H/fbzlcnKi3BDCvEe3fr3HzV5finUMX8fJspzCmPuP7fGLVO8S3UZ8RhQseFw=="
     },
     "browser-process-hrtime": {
       "version": "1.0.0",