Right now, if you type "$1" into the locked label input, it
shows up as "undefined1" in the graph TeX.

- Update the parsed content to count a lone unescaped $
  as a regular $.

Issue: https://khanacademy.atlassian.net/browse/LEMS-2591

Test plan:
`yarn jest packages/perseus/src/widgets/interactive-graphs/utils.test.ts`

Storybook
- Go to http://localhost:6006/?path=/story/perseuseditor-widgets-interactive-graph--interactive-graph-with-aria-label
- Create a new locked figure
- Add a label "$1" and confirm it shows up as "$1" in the preview
- Play around with different escaped and unescaped dollar signs
  and confirm they work as expected