fix(deps): bump gatsby-plugin-typescript from 2.1.12 to 2.1.15

Bumps [gatsby-plugin-typescript](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-typescript) from 2.1.12 to 2.1.15.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-typescript/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-typescript@2.1.15/packages/gatsby-plugin-typescript)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>