docs(interner): add code examples to boa_interner public API

[KaustubhOG]

Closes #2337

Added # Examples doc sections to all 14 public functions in
core/interner/src/lib.rs, covering Interner and JSInternedStrRef.

PR #3141 added unit tests but left the public API without doc examples.
This PR completes the original intent of the issue.

Notable considerations:

• is_empty() example reflects that a new interner is never empty due to
  pre-loaded COMMON_STRINGS_UTF8
• len() uses initial_len pattern for the same reason
• into_common() defines a local JsString enum since no standard type
  implements both From<&str> and From<&[u16]>

All 14 doctests pass (cargo +nightly test --doc).

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,505	49,505	0
Ignored	2,261	2,261	0
Failed	1,096	1,096	0
Panics	0	0	0
Conformance	93.65%	93.65%	0.00%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.10%. Comparing base (6ddc2b4) to head (c1629de).
⚠️ Report is 710 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4724      +/-   ##
==========================================
+ Coverage   47.24%   57.10%   +9.85%     
==========================================
  Files         476      553      +77     
  Lines       46892    60603   +13711     
==========================================
+ Hits        22154    34606   +12452     
- Misses      24738    25997    +1259     

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[jedel1043]

Looks good to me!

[KaustubhOG]

Looks good to me!

thanks @jedel1043 tbh i enjoyed this because it was my first PR i raised in boa