fix(runtime): stringify BigInt elements in Array.prototype.join

[proggeramlug]

What

Array.prototype.join now stringifies BigInt elements correctly. A BigInt is
NaN-boxed with BIGINT_TAG (not POINTER_TAG), so it bypassed the pointer arm in
js_array_join and fell through to the "[object Object]" catch-all. Added an
is_bigint() arm that emits ToString(BigInt) — the plain decimal digits, no n
suffix.

This also fixes Array.prototype.toString, String(array), and template-literal
interpolation of arrays, which all route through join.

Why

[10n].join() returned "[object Object]" instead of "10". Surfaced while
fixing BigInt.asUintN/asIntN ToBigInt coercion (sibling PR): test262
built-ins/BigInt/{asIntN,asUintN}/bigint-tobigint.js feeds [10n]/["10"]
through ToBigInt (→ array.toString() → BigInt parse), which silently corrupted.

Verification

Byte-identical to node --experimental-strip-types, in both PERRY_NO_AUTO_OPTIMIZE=1
and the default (codegen js_array_join_value) build:

[1n,2n,3n].join("-")  => "1-2-3"
[1n,2n,3n].toString() => "1,2,3"
String([100n, -5n])   => "100,-5"
`${[7n,8n]}`          => "7,8"
[1, 2n, "x"].join(",")=> "1,2,x"

• cargo test --release -p perry-runtime --lib green.
• Purely additive: only the previously-"[object Object]" BigInt-element path
  changes; non-BigInt arrays are untouched.