Use @babel/core#parse instead of @babel/parser#parse

[dennari]

Summary

Gatsby parses a project's source files multiple times:

1. To load the possible configuration files (e.g. gatsby-node.js)
2. To parse the GraphQL queries
3. To generate the javascript that's run on the site (through Webpack and babel-loader)

Cases 1. and 2. use @babel/parser with a hardcoded set of parser plugins (different from babel-plugin-x). Ideally, they would use the same configuration mechanism as 3 (custom .babelrc for example). There's a separate, recently added and so far undocumented, parse method in @babel/core which would allow exactly this.

Motivation

This would allow, for example, using TypeScript, or any other compile-to-js language supported by Babel, in the configuration files. It would also simplify plugins like gatsby-plugin-typescript, which wouldn't need to implement the preprocessSource api.

Here's a similar discussion in the babel-eslint project: babel/babel-eslint#573

[KyleAMathews]

Done in v2 I believe.