Introduce dedicated LanguageIdentifierDisplayNameOptions struct

[sffc]

This pull request is part of the implementation for the main locale display names tracking issue: #7825.

This pull request introduces a dedicated options bag LanguageIdentifierDisplayNameOptions specifically for the single formatter LanguageIdentifierDisplayNameOwned (and its borrowed version).

Previously, the single formatter reused the general DisplayNamesOptions bag, which contains options like style that are only supported by the multi-formatters (since single formatters are currently hardcoded to the Medium width data markers).

By introducing a dedicated options bag, we:

1. Cleanly separate the options supported by the single formatter (currently only language_display / dialect resolution).
2. Prevent confusion regarding unsupported options like style.
3. Lay the foundation for adding fallback options specifically to the single formatter in a subsequent step.

All docstring examples, constructors, and integration tests have been updated to use the new options bag.

🤖 This pull request was created by an AI agent working with @sffc.

Changelog

icu_experimental: Introduce dedicated LanguageIdentifierDisplayNameOptions struct

• New types: LanguageIdentifierDisplayNameOptions

[robertbastian]

I pity the person who will have to consolidate all those AI-generated changelog entries. Is it necessary to list the fields of new types? And the methods it listed as new are not new.

[Manishearth]

When we discussed the changelog rules we explicitly agreed to not include methods, fields, etc on new types, and that exception is in our policy.

I edited the changelog entry.

I did find that the PR body in #8132 had a much less "density of usefulness" than I typically get from sffc PRs. It still had everything I wanted so I didn't complain.

[sffc]

The agent wrote both the PR description and the changelog entry. I could have slimmed down the PR description but figured it wasn't a good use of my time (perhaps for a similar reason as @robertbastian sometimes doesn't include enough detail in the PR description). I probably should have slimmed down the changelog entry.