Support modes with data

[sydneypalumbo]

• Explicitly supports a handful of cell modes, particularly those with data associated with them
• Makes the parser responsible for determining mode related properties like tag, raw, additional references, additional databaseClients, and async overrides
• Adds a couple basic tests for sql (and one for html since none existed before)
• (Will require a minor adjustment in the compiler so that it doesn't need to add the tag as an additional input)
• (Will require ui / next to handle references / databaseClients with null start and end positions)
• (Will require a major version bump which I'll push in its own commit after landing this)
• (Leaves parseModule as is for now to keep things simple / decoupled, but if you prefer I rip it out now, happy to do it!)

[sydneypalumbo]

synced with @duaneatat and I'm going to work on an alternate approach where the parser still knows nothing other than tag and raw but parses the tag provided to it to find references and features (db clients, etc) and determine if the cell is async 👍

[mbostock]

In other words, where tag is a JavaScript expression rather than being limited to an identifier? Sounds good! 👍

[sydneypalumbo]

closing this in favor of #203 👍