Add support for React components.

[matthewp]

This adds support for react components via a new extensions config in astro.config.mjs. In the future we can extend this to do things like look at the import statements, as Snowpack does.

[natemoo-re]

Nice! Does this allow multi-part extensions like .preact.tsx and .react.tsx? Because that would make mixing similar frameworks pretty easy.

[matthewp]

Not a the moment, no. That could be done in a follow-up PR. There's an idea that Snowpack perhaps could have some sort of API like:

snowpack.getMeta('components/Example.jsx') // { framework: 'react', url: '/components/Example.js' })

Which internally would do some things such as:

1. Look at extensions config.
2. Look at file extension.
3. Look at import statements.

In some order of preference. I was hoping to avoid duplicating any code that might be better put into Snowpack.