Combined dark/light CSS files without duplicated code

[Carl-Hugo]

1.2.0 (preview)

This is a prerelease version that could contain bugs.

• Add bootstrap-dark-prefers-light.css which default to Bootstrap dark and display the light colors if the user's prefers-color-scheme: light.
• Add bootstrap-light-prefers-dark.css which default to Bootstrap and display the dark colors if the user's prefers-color-scheme: dark.
• Add bootstrap-prefers-dark-color-only.css which is only the colors of bootstrap-prefers-dark.css. This is autogenerated by clean_non_color_attr.js.
• Add bootstrap-prefers-light-color-only.css which is only the colors of bootstrap-prefers-light.css. This is autogenerated by clean_non_color_attr.js.
• Add clean_non_color_attr.js gulp plugin (based on gulp-css-remove-attributes).
• Update .NET/Razor Pages sample to use these new CSS files.

Completed state

• [x] default: light - prefers: light (bootstrap-light-prefers-dark.css)
• [x] default: light - prefers: dark (bootstrap-light-prefers-dark.css)
• [x] default: dark - prefers: light (bootstrap-dark-prefers-light.css)
• [x] default: dark - prefers: dark (bootstrap-dark-prefers-light.css)

Known issues

• Color bugs in the dark/light theme are caused by color variables being set to null in Bootstrap and overridden in Bootstrap-dark.

[Carl-Hugo]

This PR should address #17, #20, #49