Fix REPL: Handle :: operator correctly to allow multi-line input (Fixes #24142)

[vaibhav410]

Fix REPL: Handle :: operator correctly to allow multi-line input (Fixes #24142)

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🐛 Problem

The Scala 3 REPL incorrectly terminated multi-line input when the :: operator was entered on a line by itself (e.g., when constructing a list). This issue effectively broke common list construction patterns in the REPL.

Failing Behavior (Before Fix):
The REPL assumes the statement is complete and terminates the input prematurely.

scala> 1 ::
<-- REPL immediately exits multi-line input mode here


solution:
scala> 1 ::
     | 2 ::
     | Nil
val resX: List[Int] = List(1, 2)
<img width="1288" height="178" alt="Screenshot 2025-11-06 213623" src="https://github.com/user-attachments/assets/3efa6867-d98e-4460-9e0d-2f92e1b85a56" />

[bracevac]

1. To contribute, you need to sign the CLA
2. How is the problem this PR solves connected to issue #24142?
   I can successfully enter and evaluate a multi-line expression with :: on 3.7.3.
3. Generously assuming a human being created this.

[bracevac]

I fail to see how this file and change set is relevant to the issue.

[vaibhav410]

Fix for issue #24142

Hi @bracevac,
I’ve signed the Akka CLA (confirmed via email).
Also removed the unintended .gitattributes change — it was added accidentally.
This PR now only includes the REPL :: fix related to issue #24142.
Thanks for reviewing!

[SethTisue]

as the test failure in CI shows, this breaks tab completion on REPL commands such as :quit and :help and such