Require indent after colon at EOL

[odersky]

Require indented definitions after a : at the end of a line where an argument is expected.

Fixes #16452

[prolativ]

This fixes the case when there's a newline after the colon (both in REPL and when compiling an entire file) but in REPL this doesn't parse well yet if the colon is followed by the name of a parameter and an arrow and then the user presses enter, e.g.

scala> val x = Seq(1, 2, 3).map: a =>
-- Error: ----------------------------------------------------------------------
1 |val x = Seq(1, 2, 3).map: a =>
  |        ^^^^^^^^^^^^^^^^^^^
  |        not a legal formal parameter for a function literal
-- [E018] Syntax Error: --------------------------------------------------------
1 |val x = Seq(1, 2, 3).map: a =>
  |                              ^
  |                              expression expected but eof found
  |
  | longer explanation available when compiling with `-explain`

[odersky]

The REPL problem with => should be fixed now as well.