Add parent and value setter to JSONPathNode

[jg-rp]

This PR:

• Adds JSONPathNode.parent - a reference to the node's parent node.
• Changes JSONPathNode.value to be a property and setter. Now, when assigning to JSONPathNode.value, source data is updated too.
• Updates the compliance test suite.
• Adds Python 3.14 to the test matrix.

See #21