Improve Frontmatter Parsing (gatsby-mdx support)

[renvrant]

Issue opened to keep an eye on this as it develops. It may ultimately be a feature in Gatsby-MDX.
ChristopherBiscardi/gatsby-mdx#352

[renvrant]

This conversation is still ongoing but I've yet to see any working solutions mentioned elsewhere. Continuing to keep an eye on it as I'm thinking the current way is probably causing slow build times.

Discussion:
gatsbyjs/gatsby#18570