feat(lookup-spa): add lookup-spa template

[mjg123]

Description

This template creates a front end as assets and a function which lets
folks use the Lookup API from their browser.

Checklist

• I ran npm test locally and it passed without errors.
• I acknowledge that all my contributions will be made under the project's license.

Notes

This template contains a compiled and minified JavaScript file in assets/lookup-spa-dist.js. The source for this is under assets-src and the config and npm run script are included in package.json, webpack.config.js and .babelrc.

If any of the source in assets-src is changed, then lookup-spa-dist.js should be regenerated as part of the same changeset. Do this by running npm install && npm run build-assets in the template root.

As I understand it, this would have to be a manual process at the moment.

[mjg123]

Tagging @dkundel and @philnash to discuss the implications of having a build-step as part of this template. I'm happy to take your advice here - as I see it there's a few options:

1. Keep this as-is and rely on the manual process
2. Rewrite this template to not need the build step
3. Add some kind of hook to run npm run build-assets automatically

Option 1 seems risky. I'm not keen on option 2, personally. I think 3 adds flexibility for building larger apps as templates. But I don't know anything about where or how to add such a hook, so this is totally open for discussion.

[mjg123]

Side question: Was I supposed to include the .env file in my template? I didn't, because it's ignored in the top-level .gitignore, this suggests that maybe I should have https://github.com/twilio-labs/function-templates/blob/main/docs/CONTRIBUTING.md#adding-environment-variables

[mjg123]

Rebased to latest main and updated the code after @philnash reviewed it (Thanks!). I see that .env has been removed from .gitignore now so I have added it to this PR.