feat: Update theme-ui to v0.3.x

[LekoArts]

Theme UI updated to v0.3 a while ago and I now finally had time to go through the breaking changes: https://theme-ui.com/migrating#v03

In general this update should be painless and without any breaking changes (for most people). The sites look and function identical, it was more a technical refactoring.

Breaking Changes

These changes might be breaking for some people:

• @lekoarts/gatsby-theme-specimens: Renamed the theme prop on the <Heading /> component to config
• @lekoarts/gatsby-theme-specimens & @lekoarts/gatsby-theme-styleguide: Removed theme-ui as a direct dependency and moved them to peerDependencies. This supersedes the PR fix(deps): make theme-ui a peerDependency #262

Description

• Updated all gatsby, react, react-dom packages to latest version
• Removed @emotion/core and @mdx-js/react packages as theme-ui now comes with it
• Removed <Styled.root /> components as Theme UI now places that on the body by default
• Replaced Styled.x with e.g. Link, Heading
• Moved @emotion/core, @mdx-js/mdx & @mdx-js/react to yarn resolutions to only have one version of each dependency
• Use merge from theme-ui for config merging
• Clean up <Global /> by moving stuff to the Theme UI config
• Rename some internal TypeScript types
• Add/Update some comments
• Use optional chaining.?

[LekoArts]

Successfully published:
 - @lekoarts/gatsby-theme-cara@1.2.0
 - @lekoarts/gatsby-theme-emilia-core@1.3.0
 - @lekoarts/gatsby-theme-emilia@1.3.0
 - @lekoarts/gatsby-theme-emma-core@1.3.0
 - @lekoarts/gatsby-theme-emma@1.4.0
 - @lekoarts/gatsby-theme-graphql-playground@1.1.0
 - @lekoarts/gatsby-theme-minimal-blog-core@2.3.0
 - @lekoarts/gatsby-theme-minimal-blog@2.3.0
 - @lekoarts/gatsby-theme-specimens@2.2.0
 - @lekoarts/gatsby-theme-status-dashboard@1.1.0
 - @lekoarts/gatsby-theme-styleguide@1.1.0