Add default themes/palettes

[innocenzi]

I'd like to add default themes so I can have something ready to use for every new project instead of having to either define a palette or copy a previous one.

For instance, I'd like to be able to require the plugin this way:

plugins: [
  require('tailwindcss-theming/themes/default')
]

For the default themes, I plan to deliver the following ones:

• Tailwind 1 colors (themes/legacy?)
• Tailwind 1.2 colors (the new palette Adam and Steve are working on, as themes/tailwind)
• A custom light/dark theme based on Tailwind 1.2 colors (themes/default)

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I'll wait for the official release of the new Tailwind color palette (it's actually in @tailwindcss/ui currently, but prone to changes) before adding this.

[innocenzi]

See #22