Add ability to detect if theme is in "dev" mode

[BryceRussell]

Currently astro-theme-provider runs all code for both the author and user, but in some cases code only needs to be executed while authoring/developing a theme.

astro-theme-provider could solve this by checking for the existence of a package.json inside a playground directory. A theme package already has to be developed inside of a monorepo with a playground for type generation so checking for the existence of the playground's package.json should be a safe way of validating that a theme is running in "dev" mode since this file wont be included in the final output of the package.

export default defineTheme({
  dev: '../playground' // default value
})