Currently Prism returns `42` for code like this:
```ruby
42.tap { it = it; p it } # => 42
```

But parse.y returns `nil`:
```ruby
42.tap { it = it; p it } # => nil
```

In parse.y, it on the right-hand side is parsed as a local variable.
In Prism, it was parsed as the implicit block parameter it, which caused this inconsistent behavior.

This change makes the right-hand side it to be parsed as a local variable, aligning with parse.y's behavior.

Bug ticket: https://bugs.ruby-lang.org/issues/21139

https://github.com/ruby/prism/commit/cf3bbf9d2c