Fix union of quoted property names returning no results

[spokodev]

Problem

A bracketed union of quoted property names — the canonical Goessner /
RFC-9535 form $['x','y'] — returns [] instead of the selected values:

JSONPath({ path: "$['x','y']", json: { x: 1, y: 2, z: 3 } });
// ACTUAL:   []
// EXPECTED: [1, 2]

Same for $["x","y"] and $['x', 'y']. The non-standard bare form $[x,y]
works ([1, 2]), and numeric unions $[0,2] work — only the quoted (standard)
form is broken.

Root cause

toPathArray only strips the surrounding quotes for a single-name bracket
(['name'], via /\[['"]([^'\]]*)['"]\]/). A multi-member bracket ['x','y']
doesn't match that pattern, so the interior quotes survive tokenization:

toPathArray("$['x','y']") → ["$", "x','y"]

The _trace union branch then does "x','y".split(',') → ["x'", "'y"] and
looks up properties literally named x' and 'y, which don't exist → [].

Fix

Strip a surrounding quote from each comma-separated member before the lookup.
Bare and numeric members carry no quotes, so they are unaffected (the strip is a
no-op for them).

Test

Added a "Union of quoted property names" case to test/test.properties.js
covering $['x','y'], $["x","y"], $['x', 'y']. Fails on main, passes with
the fix; the existing suite (280 tests) is unchanged.

Closes #159.