Use get-unique-selector instead of Nyxt IDs

[aartaka]

Is your feature request related to a problem? Please describe.
Our current nyxt-identifier-reliant page interaction model is quite shaky and not-exactly-privacy-preserving.

Describe the solution you'd like
An alternative technique that I've been drafting and the prototype of which is already in nyxt/dom: get-unique-selector. Computing the unique for the element is:

• Possible.
• Resilient to minor page modifications.
• And privacy-preserving (no Nyxt-specific DOM modifications).

Describe alternatives you've considered
Possibly reuse WebKit APIs for DOM (but these are extension-specific and deprecated in latest versions?)

Additional context
See 9cabd48#commitcomment-112564009 for the initial discussion.
finder can be an inspiration and a template for a complete implementation.