Fix import issue with React InertiaHeadProps type

[jessarcher]

In #1448, the InertiaHeadProps type was updated to include React's children prop.

While the adapters built fine, this caused an issue when building the React playground:

../../packages/react/types/Head.d.ts:1:8 - error TS1259: Module '"/home/jess/Code/inertiajs/inertia/node_modules/@types/react/index"' can only be default-imported using the 'allowSyntheticDefaultImports' flag

1 import React, { FunctionComponent } from 'react';
         ~~~~~

Although #1448 didn't change the import of React in Head.ts, the change did result in the generated types for that file now also needing to import React when it previously didn't.

Rather than use the allowSyntheticDefaultImports flag, I've changed the InertiaHeadProps type to not depend on the React import directly. Also, rather than manually typing the children prop, I've used React's PropsWithChildren type which has the same end result.

[jessarcher]

This error doesn't seem to occur any longer, and I've merged #1647, which adds the playground builds to CI.