Support multiline collective extensions in REPL

[nicolasstucki]

scala> extension (x: Int) def f =
     | 0
def extension_f(x: Int): Int

scala> extension (x: Int)
1 |extension (x: Int)
  |                  ^
  |                  Extension without extension methods

scala> extension (x: Int):                                                           
1 |extension (x: Int):
  |                   ^
  |                   Extension without extension methods

scala> extension (x: Int) {
1 |extension (x: Int) {
  |                    ^
  |                    `def` expected

scala> extension (x: Int) { def f = 0 }
def extension_f(x: Int): Int

[bjornregnell]

Workaround: wrap it in an object and use Alt+Enter:

scala> object A:
     |   extension (i: Int)
     |     def a = i + 1
     |     def b = i + 2
     | 
// defined object A

scala> import A.*

scala> 12.a                                                                     
val res0: Int = 13

scala> 12.b                                                                     
val res1: Int = 14

But this should work on top level (using Alt+ENTER to create new line):

scala> extension (i: Int) 
     |   def a = i + 1
     |   def b = i + 2
     |  
4 | 
  | ^
  |The start of this line does not match any of the previous indentation widths.
  |Indentation width of current line : 1 space
  |This falls between previous widths: 0 spaces and 2 spaces

[allanrenucci]

The REPL relies on the parser emitting an incomplete input error.

Replacing the following call to syntaxError by a call to syntaxErrorOrIncomplete should fix the issue for the first two examples. Not sure about the last one. https://github.com/lampepfl/dotty/blob/cd461024456b937a90adfe7f0b18798a5c8c260f/compiler/src/dotty/tools/dotc/parsing/Parsers.scala#L3652