browser_only: don't convert to runtime errors on identifiers or function application

[jchavarri]

I had a case where I used browser_only in some event handler using a curried function, e.g.:

onKeyDown=[%browser_only
  onKeyDown(~foo)
]

This code converted to plain Runtime.fail_impossible_action_in_ssr, crashing the server application at runtime.

This PR removes some of the cases where browser_only can be used:

• identifiers
• function application (pexp_apply)

so this extension can only be used to convert function definitions.

Users can still use switch%platform for identifiers and function applications, which prevents running on footguns like the one mentioned above.