Add ShadowRoot support (GetShadowRoot and find from shadow)

[redforks]

Implement W3C WebDriver ShadowRoot support.

This adds:

• Add method Element::shadow_root()
• ShadowRoot struct
• ShadowRoot::{find, find_all}() methods

This enables locating elements inside open Shadow DOM trees,
allowing Fantoccini to interact with modern Web Components
(e.g. Shoelace, Lit, Vaadin).

I need to e2e test a web app uses shoelace web component, and this patch works

[jonhoo]

No notes, thank you!

[jonhoo]

Ah, I would prefer if you could add a test for this functionality please. But will land without one for now.

[codecov]

Codecov Report

❌ Patch coverage is 10.14493% with 62 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.71%. Comparing base (77f03d4) to head (b1c4259).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/elements.rs	0.00%	59 Missing ⚠️
src/session.rs	70.00%	3 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/session.rs	82.53% <70.00%> (-1.35%)	⬇️
src/elements.rs	49.85% <0.00%> (-9.88%)	⬇️

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[jonhoo]

Released in 0.22.1 🎉