Add workflow to build css and js files

[IanDelMar]

Description

This PR adds a workflow that runs the copy-assets script and rebuilds CSS and JS files every time a node package, a js file or an scss file changes.

Motivation and Context

• When dependabot opens a pull request it's not immediately clear if updating this dependency will have effects on the build scripts or the build outcome. Automatically running scrips and rebuilding files shows such effects right away.
• If someone opens a pull request proposing a change to an scss file but forgets to rebuild the css files this will happen automatically.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (a code change that neither fixes a bug nor adds a feature)
• Styling (changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc))
• Development (changes that affect the build system or external dependencies)
• Documentation (changes that affect existing inline documentation)

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have read the CONTRIBUTING document.

[IanDelMar]

@bacoords Ok for v1.2.3

[bacoords]

Nice - I'm going to merge it into develop but also want to keep an eye on it with the other PRs that come in to see how well it works.