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MINOR: [C++] Separate ListArray from LargeListArray member function implementations (#36276)

### Rationale for this change

Grouping the functions by the class they belong to make it less confusing when I need to add a third and fourth list class.

### What changes are included in this PR?

Just moving functions around in the source file.

### Are these changes tested?

Yes.

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>