feat: Configurable widths & themes

[michaeltaranto]

This PR adds the ability to configure which themes and widths are viewable, and store them as a local user preference. In addition this preference is encoded in the url, so when sharing a link, consumers see the same setup as the author.

Also note, with the addition of a toolbar to house the preference menus, we have removed the option to left dock the editor. It already felt strange, but with these new features it really detracted from the overall experience of the preview and code taken precedence.

Features

• Configurable themes & widths as user preference
• Configurable themes & widths in share url
• Removed left docked editor option

Bug Fixes

• Changed order of webpack module resolution extensions array:

-['.mjs', '.js', '.json', '.ts', '.tsx']
+['.tsx', '.ts', '.jsx', '.js', '.json']

Should fix #105.

Refactor

• Hoisted fonts, colours, shadows, etc into top level theme/variables.

[markdalgleish]

Awesome work 👏👏👏