Better handling of leading infix operators in indented code

[odersky]

@main def Test =
  val x = false
  val y = 1
  val result =
    x
    || y.match
        case 1 => false
        case 3 => false
        case _ => true
    || !x
  assert(result)

In this code, the last || !x was seen as a part of the previous case, so the code was parsed as

@main def Test =
  val x = false
  val y = 1
  val result =
    x
    || y.match
        case 1 => false
        case 3 => false
        case _ => true || !x
  assert(result)

This is highly surprising and unintuitive. The fix will insert an <outdent> token instead
if the leading infix operator is too far to the left. Too far means: (1) left of the current indentation
region, (2) and not to the right of any outer indentation widths.

(2) allows to still parse code like this

if xyz then
    one
  + two
  + three

Here, the width of the indentation region after then is 4, but the + operator is to the right
of the outer indentation width of 0., so the indentation region is not closed. In other words,
we do not close an indentation region if the result would not be legal, since it matches none of
the previous indentation widths.

[liufengyun]

LGTM

[liufengyun]

The following code compiles previously:

class Test:
   if
     5 < 6
   + 3
   then
    println("hello")

But I guess it's fine to reject it after the change.

[odersky]

I think it's OK, in particular since there is no legacy code, since leading infix operators are a Scala 3 thing.