tools: ynl-gen: rename TypeArrayNest to TypeIndexedArray

tools: ynl-gen: rename TypeArrayNest to TypeIndexedArray

Since TypeArrayNest can now be used with many other sub-types
than nest, then rename it to TypeIndexedArray, to reduce
confusion.

This patch continues the rename, that was started in commit
aa6485d813ad ("ynl: rename array-nest to indexed-array"),
when the YNL type was renamed.

In order to get rid of all references to the old naming,
within ynl, then renaming some variables in _multi_parse().

This is a trivial patch with no behavioural changes intended.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250915144301.725949-8-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>