Support schema presets block type completion

[aswamy]

What are you adding in this PR?

Fixes #628

Add support for completion of presets block type.

• Look at the blocks section (whether it be locally-defined blocks or block references) to auto-complete block types in the preset area
• Preset blocks can be nested, so we need to fetch block files and read its schema to determine what to auto-complete

Testing

• Checkout Shopify/dawn
• Find a section where it has section blocks defined under schema
• In preset, add a blocks and see if type gets auto-completed by whatever is in blocks section

(see all edge cases in the ticket)

What's next? Any followup issues?

#624

Before you deploy

• I included a minor bump changeset
• My feature is backward compatible