src: general C++ cleanup in node_url.cc

- Merge `domain` and `opaque` storage in URL parser:

  This just simplifies the code a bit, having multiple fields
  in an union with the same type is usually just overhead.

- Add move variant of `URLHost::ToString()`:

  This helps avoid unnecessary string copy operations, especially
  since we control the lifetime of `URLHost` objects pretty well.

- Use const refs in node_url.cc where appropriate

- Remove or reduce overly generous `.reserve()` calls:

  These would otherwise keep a lot of unused memory lying around.

- Return return values instead of unnecessary pointer arguments

- Use more common/expressive variable names

- Avoid macro use, reduce number of unnecessary JS strings:

  There’s no reason for `GET`, `GET_AND_SET` and `UTF8STRING` to be
  macros. Also, `GET` would previously create a JS string instance
  for each single call, even though the strings it was called
  with were compile-time constants.

- Avoid unnecessary JS casts when the type of a value is known

- Avoid (commonly unnecessary) copy for whitespace stripping

PR-URL: https://github.com/nodejs/node/pull/19598


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>