feat: add a `parent` property to AST nodes

[RomFouq]

• Change the way HTML text nodes are stored: use objects instead of strings since strings cannot accept new properties
• Populate the parent property when generating the AST

Partially addresses #81.

[cossssmin]

@Scrum this would be helpful, no?

[Scrum]

@Scrum this would be helpful, no?

Not obvious potential that can complicate the system