extensions-tool: Escape '\' and '"' in description

If description contains \ and/or " in its description, extensions-tool generates invalid json like below:

{
  "name": "foo",
  "description": "foo "bar" baz",
  "uuid": "foobar@example.com",
  "shell-version": [
    "3.36"
  ]
}

This fixes that by escaping \ and ".