Minor.Request Context Storage Methods

BREAKING CHANGE: Remove context.storage, context.session, context.sessionStarted, context.formData, and context.files from @remix-run/fetch-router, and rename createStorageKey(...) to createContextKey(...).

RequestContext now provides request-scoped context methods directly (context.get(key), context.set(key, value), and context.has(key)), using keys created with createContextKey(...) or constructors like Session and FormData.

Session middleware now stores the request session with context.set(Session, session), and form-data middleware now stores parsed form data with context.set(FormData, formData). Uploaded files are read from context.get(FormData) using get(...)/getAll(...).

RequestContext is now generic only over route params (RequestContext<{ id: string }>), and no longer accepts a request-method generic (RequestContext<'GET', ...>).