The current allowlist for capitalized function identifiers only allows stdlib JS
modules. This PR introduces a new compiler option to allow passing in a set of
allowed capitalized user function identifiers. It's a hack (in the absence of a
type system) to let us check that capitalized function calls are only possible
for identifiers that aren't bound to a React component.
I considered adding a mechanism in fixtures to configure compiler options
per-fixture, but opted to keep it simple for now and special case a
`ReactForgetSecretInternals` identifier as one allowed user function in
transform tests.