Check trailing blank line at EOF for OUTDENT

[som-snytt]

Boost the previous fix for "spurious indentation error for blank line at EOF" by checking that the line is a non-empty blank line.

This accommodates REPL, which routinely parses lines at EOF.

Also improve the previous fix for "correctly detect colon arrow at EOL" by returning token EOF at EOF. The check for EOF when probing whether followingIsLambaAfterColon was specifically to accommodate REPL.

Fixes #22844

[som-snytt]

An adjustment is required.

scala> println:
     |   42
     |
42

scala> List(42).map: x =>
     |   x + 1
     |
val res0: List[Int] = List(43)

scala> List(42).map: x =>
-- Error: ----------------------------------------------------------------------
1 |List(42).map: x =>
  |^^^^^^^^^^^^^^^
  |not a legal formal parameter for a function literal

scala>

where res0 was from history, and the error shows "arrow eol" is still broken.

Also the scripted test does not actually pass.

In incomplete state, REPL re-parses on every character (IIUC); I expected to follow up with other improvements in that area.

[som-snytt]

Dotty ReplTest does only single-line inputs; at least "scripted" test should work like Scala 2 SessionTest.

The gold standard integration test would run a REPL process from test input, in order to test incomplete input:

List(42).map: x =>// at eof
List(42).map: x =>\n// at eof
List(42).map: x =>\n  x+1// at eof
List(42).map: x =>\n  x+1\n\n // complete

The JLine parser could be tested for internal behavior, but scripted or "session" tests are easier to create and read.