Update custom elements manifest

[clhenrick]

Makes some updates to files to more correctly document the custom element using the custom elements manifest

recreates / updates the custom-elements.json with the following changes:

• docs for CSS variables
• docs for two slots
• docs for component summary
• ignores documenting private class members on theColorLegendElement class
• ignores documenting non-essential modules (everything but color-legend-element.ts and constants.ts)

Other:

• updates the package.json's files field to include custom-elements.json in the package root when publishing
• updates the package.json's files field to ignore the build/tests/ directory so that it's excluded when publishing

[clhenrick]

LGTM