[Kotlin] getSignature returns undefined due to getChildByField on fieldless grammar

[hiyizhiyu]

Bug Description

tree-sitter-kotlin does not use field names, but getSignature
calls getChildByField(node, 'function_value_parameters'),
which causes the signature of Kotlin methods to always resolve
to undefined.

Fix

The fix is consistent with extractKotlinReturnType:
replace getChildByField with iteration over namedChildren,
matching the target node by its type property.