Correctly handle numeric separators with leading zeros

[bishabosha]

minimized code

val x = 0_1
val y = 0_00.1

... compiles fine

expectation

1 |val x = 0_1
  |        ^
  |Non-zero numbers may not have a leading zero.

1 |val y = 0_00.1
  |        ^
  |Non-zero numbers may not have a leading zero.

scala 2 does not allow the definition of y, dotty does, despite not allowing more than one leading zero for a non zero number.

Scala 2 is not opinionated about handling of x, but has no restrictions on leading zeros. Because dotty does not allow leading zeros for integers that are not zero, then separators should not change this behaviour