[EPIC] Ibis expression API stability

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Acceptance criteria: the Ibis user-facing expression API is stabilized for 10.0+. While we will always reserve the right to make breaking changes, we are now in the place where stability is important for production dependencies. Organizations choosing Ibis should feel confident their code will work with minimal effort when upgrading to new versions.

TODOs:

P0

dtype or type or cast

• bug: many Operations can't handle NULL optional arguments #8833

P1

refactor

• refactor: deprecate NA/fillna/dropna in favor of name that uses null instead of na #7276
• feat(api): standardize the position of null values in sorted columns and provide an api for specifying null order #7252

dtype or type or cast

• feat: allow specifying Optional and/or T | None in UDFs #7133
• Possibility to extract isoyear and isoweekday (or weekday) from a timestamp column. #8989 (draft PR open feat(api): isoyear method #9034)
• feat: Support type arg to ibis.array and ibis.map #8289 (PR open feat: improve map(), struct(), array() #8666 needs work)

other

• refactor: standardize use of keyword/positional-only arguments #9125

P2

refactor or deprecate or breaking changes

• refactor(ux): ensure that all known and knowable type errors raise UnsupportedBackendType instead of letting code pass through to the backend #8333

dtype or type

• bug: dt.highest_precedence([dt.uint64, dt.int32]) gives uint64, not int64 #7331

other

• feat: consolidate ibis.case(), Value.case(), Value.cases(), Value.substitute() #7280 (PR in progress feat(api): move from .case() to .cases() #9039)