[Feature]: Support for distinguishing between SSG and CSR environments

[hotoo]

What problem does this feature solve?

My website has SSG enabled, but there's a custom component that I don't want to render during SSG build to static HTML. Instead, I want it to automatically calculate and determine whether to display based on environment information during CSR.

What does the proposed API look like?

Provide an environment variable or other solution to determine whether it's an SSG environment in MDX or React components?