feat(datetime): Add YearStyle::Always and YearStyle::Never

[mega123-art]

Adds YearStyle::Always and YearStyle::Never options as per TC39 intl-eradisplay proposal.

Fixes #6010

[sffc]

Always is the same as WithEra and is not needed.

Never would be a useful addition, but it seems like not a good addition to the enum which is named YearStyle, not EraDisplay as the ECMA proposal.

Also please note the "implied" style in the ECMA proposal.

[mega123-art]

Always is the same as WithEra and is not needed.

Never would be a useful addition, but it seems like not a good addition to the enum which is named YearStyle, not EraDisplay as the ECMA proposal.

Also please note the "implied" style in the ECMA proposal.

Thanks for the review! After investigating the TC39 proposal-intl-eradisplay, I found:

Current proposal status (Stage 2):

• Only defines 3 values: "never", "always", "auto"
• The "implied" option you mentioned doesn't exist in the current spec yet

Issues with my implementation:

• Agree: Always is redundant with WithEra
• Agree: Never belongs in an EraDisplay enum, not YearStyle
• The current YearStyle mixes year precision (2-digit vs 4-digit) with era display

Proposed path forward:
Would it make more sense to:

1. Wait for TC39 proposal to reach Stage 3+ before implementing?
2. Create a separate EraDisplay option instead of extending YearStyle?

Should I close this PR for now? Should I update issue #6010 with these findings?

[sffc]

We would accept a PR adding something like YearStyle::NoEra, corresponding to the ECMA-402 proposal eraDisplay: "never".

There is discussion of eraDisplay: "implied" in the ECMA-402 proposal repository.

[mega123-art]

@sffc sir i tried to make those suggested changes

[sffc]

Suggestion (optional): Split this test into tests on each variant, and replace the top-level docs here with a table summarizing the results.