Refuse an array shape pljs cannot represent, instead of changing the value

[sfc-gh-okalaci]

Stacked on #42.

pljs represents a SQL array as a flat JavaScript array. Both directions handled an array
that does not fit that model by quietly producing something else.

Reading — deconstruct_array() flattens, so a multidimensional array arrived in
JavaScript one-dimensional. {{1,2},{3,4}} became [1,2,3,4], and writing it back gave a
different value than was read.

Writing — a nested JavaScript array aimed at a scalar element type went into the array
conversion anyway, because the dispatch condition asked only whether the target was
json/jsonb, which is false for every scalar. The element loop then converted each inner
array to the element type:

CREATE FUNCTION f() RETURNS int[] AS $$ return [[1,2],[3,4]]; $$ LANGUAGE pljs;
SELECT f();
 {-1629977232,-1629977184}

Those are the inner arrays' ArrayType pointers reinterpreted as int4. No error, and
nothing about the output says it isn't data.

Reading now raises ERRCODE_FEATURE_NOT_SUPPORTED for a multidimensional array; writing
raises ERRCODE_DATATYPE_MISMATCH naming the target type. One-dimensional arrays are
unaffected in both directions, and a nested array remains valid for json/jsonb — the
targets that can actually represent one.

Test plan

sql/pg_array_shape.sql covers both directions, both error paths, a scalar target
(int, text), that one-dimensional arrays still round-trip, and that nested arrays
still work for jsonb and jsonb[].

It discriminates: reverted, the read silently returns [1,2,3,4] and the write returns
{-1629977232,-1629977184}. Full suite green on PostgreSQL 17 (55 tests); builds clean on
16, 18 and 19beta3.