Allow import language.experimental.captureChecking in the REPL

[mbovel]

Attempt to fix #16250 with @rjolly during the issue spree.

Two things currently seem to prevent import language.experimental.captureChecking to be used in the REPL.

1. needsCaptureChecking flag preservation across compilation units

The needsCaptureChecking is defined here:

scala3/compiler/src/dotty/tools/dotc/CompilationUnit.scala

Line 57 in b5941e4

var needsCaptureChecking: Boolean = false

And is set from handleGlobalLanguageImport:

scala3/compiler/src/dotty/tools/dotc/config/Feature.scala

Line 153 in b5941e4

ctx.compilationUnit.needsCaptureChecking = true

To preserve it, we tried calling handleGlobalLanguageImport in setImportInfo (4702968).

2. outermost syntax errror

Currently, a syntax error is emitted if the import is not at the top level:

scala3/compiler/src/dotty/tools/dotc/parsing/Parsers.scala

Lines 3312 to 3313 in b5941e4

	if Feature.handleGlobalLanguageImport(prefix, imported) && !outermost then
	syntaxError(i"this language import is only allowed at the toplevel", id.span)

Outermost is originally set here:

scala3/compiler/src/dotty/tools/dotc/parsing/Parsers.scala

Line 764 in b5941e4

val isOutermost = in.currentRegion.isOutermost

We are not sure yet if and how we should special-case this flag so that it works with wrapper objects generated by the REPL.

[mbovel]

Closing in favor of the new #21725.