Fix incorrect "field not initialized" error with try/else

[SeanTAllen]

Previously, due to a bug in the compiler's while/else handling code, the
following code would be rejected as not initializing all object fields:

actor Main
  var _s: (String | None)

  new create(env: Env) =>
    try
      _s = "".usize()?.string()
    else
      _s = None
    end

This commit fixes try/else handling in refer_try so that it matches the
general shape of refer_if, refer_while, and refer_repeat. It differs from
them because try isn't a scope and we want to push everything into the then
clause as it can fulfill initialization all on its own or if it doesn't handle
initialization, we can still get it from the try body and the else clause.

Closes #3283

[ergl]

Assigned myself to these ones just so I can double check how we do this for if-then-else, but if anyone else with more expertise with the compiler thinks these PRs lookg good, feel free to merge them.

[ergl]

@SeanTAllen This needs to be rebased after merging #4009