Enforce indentation off-side rule

Ref lampepfl/dotty 10671

Currently

class A:
  val x = 1
    val y = 2

is admitted. This becomes confusing since this looseness ends up not
catching things like

class Test:
  test("hello")
    assert(1 == 1)

where `assert(1 == 1)` is actually not passed into `test(...)(...)`.

Following the convention of indentation language like Python, we should
reject superfluous indentation, which can be defined as having two or
more indentation widths within a region.

Author: eed3si9n

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -496,11 +496,16 @@ object Scanners {
       currentRegion match {
         case Indented(curWidth, others, prefix, outer)
         if curWidth < nextWidth && !others.contains(nextWidth) && nextWidth != lastWidth =>
-          if (token == OUTDENT)
+          if token == OUTDENT then
             errorButContinue(
               i"""The start of this line does not match any of the previous indentation widths.
                   |Indentation width of current line : $nextWidth
                   |This falls between previous widths: $curWidth and $lastWidth""")
+          else if others.nonEmpty then
+            errorButContinue(
+              i"""Unexpected indentation:
+                  |Indentation width of current line: $nextWidth
+                  |Expected indentation             : ${others.head}""")
           else
             currentRegion = Indented(curWidth, others + nextWidth, prefix, outer)
         case _ =>