Allow global language imports in REPL and snippet compiler

[bracevac]

Previously, import language.experimental.captureChecking (and related global language imports) were rejected in the REPL and snippet compiler because they appeared in nested scopes rather than at the toplevel.

This change takes a structural approach to fix both contexts:

• REPL: Forward outermost through blockStatSeq in Interactive mode so the parser treats REPL-level imports as toplevel. After a successful compile, propagate global language imports to rootCtx as -language: settings so subsequent inputs can parse CC syntax (e.g. ^).

• Snippet compiler: Extract global language imports from snippet body and place them before the object Snippet {} wrapper, making them genuine toplevel imports.

Fixes #16250

How much have your relied on LLM-based tools in this contribution?

Used Claude extensively to analyze the problem and iterate on the solution.

How was the solution tested?

There are REPL and snippet compiler tests. I also verified manually
within the REPL that the top-level CC import works.
Updated some snippets in the CC language ref to verify that the snippet compilation
works as well.

[bracevac]

This is an alternative to #25415

[natsukagami]

Function types seem to have wrong capture sets printed:

scala> import language.experimental.captureChecking

scala> def f(x: () -> Unit): Unit = x()
def f(x: () -> Unit): Unit

scala> class A extends caps.SharedCapability
// defined class A

scala> val t = () => { a; () }
1 warning found
-- [E129] Potential Issue Warning: ---------------------------------------------
1 |val t = () => { a; () }
  |                ^
  |                A pure expression does nothing in statement position
  |
  | longer explanation available when compiling with `-explain`
val t: () -> Unit = Lambda/0x00007f7cc067c208@49ed96e3

scala> f(t)
-- [E007] Type Mismatch Error: -------------------------------------------------
1 |f(t)
  |  ^
  |  Found:    (t : () ->{a} Unit)
  |  Required: () -> Unit
  |
  |  Note that capability `a` cannot flow into capture set {}.
  |
  | longer explanation available when compiling with `-explain`
1 error found

[bracevac]

Yikes! I didn't realize how broken the CC pretty printing was. Maybe this is worth its own follow-up PR?

Edit: It was straightforward to fix.

[natsukagami]

Nicer error messages too 🎉