Feature: Emit utility types for Gatsby

[cometkim]

• PageProps: Enhanced support for page queries/components #74
• PluginOptionsSchema: Generate type definitions from pluginOptionsSchema #121
• gatsby-node: Types for gatsby-node.ts #110

[cometkim]

Gatsby provides a utility type PageProps contains Data and Context type parameters describe what injected into the page component.

Data can be the generated type from page queries, and Context is a subtype of the SitePageContext type.

The plugin knows what is the type for each page component and can automatically generate props type definitions.

#74