feat(extensions): add subscript_operator and index_of functions

[benbellick]

Adding two common list functions to the core spec.

subscript_operator

Returns the element at a 1-based index, or NULL for out-of-bounds access. Matches PostgreSQL and CockroachDB semantics.

In an earlier iteration, I tried to get this to cover more databases (e.g. Trino) but it was becoming an hodgepodge of configuration parameters, so I opted to just have the simple impl for now. We can add more complexity later if necessary.

index_of

Returns the 1-based index of the first occurrence of a value in a list, or NULL if not found. Matches PostgreSQL, DuckDB, DataFusion, and CockroachDB.

Trino returns 0 instead of NULL when not found, which is... strange. Just going to not have Trino's behavior represented here.

Verification script

# subscript_operator - PostgreSQL
docker run --rm postgres:latest su postgres -c \
  "initdb -D /tmp/d >/dev/null 2>&1 && pg_ctl start -D /tmp/d -l /tmp/l -o '-k /tmp' >/dev/null 2>&1 && sleep 2 && psql -h /tmp -t -c \"
    SELECT (ARRAY[1,2,3])[2], (ARRAY[1,2,3])[0], (ARRAY[1,2,3])[5], (ARRAY[1,2,3])[-1];
  \""
# Output: 2 | | |
# (empty = NULL)

# subscript_operator - CockroachDB
docker run --rm cockroachdb/cockroach:latest demo --no-example-database --insecure -e "
  SELECT (ARRAY[1,2,3])[2], (ARRAY[1,2,3])[0], (ARRAY[1,2,3])[5], (ARRAY[1,2,3])[-1];
" 2>&1 | tail -3
# Output: 2 | NULL | NULL | NULL

# index_of - PostgreSQL
docker run --rm postgres:latest su postgres -c \
  "initdb -D /tmp/d >/dev/null 2>&1 && pg_ctl start -D /tmp/d -l /tmp/l -o '-k /tmp' >/dev/null 2>&1 && sleep 2 && psql -h /tmp -t -c \"
    SELECT array_position(ARRAY[1,2,3], 2), array_position(ARRAY[1,2,3], 5);
  \""
# Output: 2 |
# (empty = NULL for not found)

# index_of - DuckDB
docker run --rm duckdb/duckdb:latest duckdb -noheader -list -c "
  SELECT list_position([1,2,3], 2), list_position([1,2,3], 5);
"
# Output: 2|NULL

# index_of - CockroachDB  
docker run --rm cockroachdb/cockroach:latest demo --no-example-database --insecure -e "
  SELECT array_position(ARRAY[1,2,3], 2), array_position(ARRAY[1,2,3], 5);
" 2>&1 | tail -3
# Output: 2 | NULL

Closes #967

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This change is 

[yongchul]

I believe #917 is proposing 0-based index 😆 I believe I commented whether we should offer an option to do either zero-based index or one-based index... So, we'd better be consistent.