Add initial code

[alexgibson]

Description

• Adds banner JS logic / tests.
• Adds banner SCSS/CSS.
• Adds webpack for building NPM package and local dev environment.
• Updates documentation.

Testing

Start with README.md and take things from there.

• Does everything install OK?
• Do the docs make sense?
• Does the demo / local dev page work correctly running npm start?
• Does building the package using npm run build work OK?
• Do the tests pass as expected running npm test?

[stephaniehobson]

🦆 🦆 🦆 Ducks are all in a row. Just one blocking issue and a bunch of other thoughts that you are under no obligation to do if you are sick of looking at this code ;)

[alexgibson]

@stephaniehobson many thanks for the review - I pushed a commit with some fixes 👍

[alexgibson]

@stephaniehobson could you take a look and approve this PR if the updates look OK? Thank you

[stephaniehobson]

R+ 🦆

[stephaniehobson]

Leaving it for you to handle the squash and merge.