Add the LINQ operators Query.jl is missing

[davidanthoff]

Third of four PRs. Adds the user-facing macros and documentation for the 38 operators in QueryOperators.jl#56, closing the gap between Query.jl's operator set and System.Linq.Enumerable as it stands at .NET 11.

CI will stay red until QueryOperators 1.2.0 is registered. The [compat] bound is an ordinary QueryOperators = "1.2"; no feature guards. Locally the full suite passes: 34 test items (18 new) plus 20 new doctests.

What lands

@left_join @right_join @full_join · @concat @union @union_by @except @except_by @intersect @intersect_by · @take_while @drop_while @take_last @drop_last · @order @order_descending @reverse @shuffle @index · @append @prepend @zip · @count_by @aggregate_by @chunk · @of_type @cast · @min_by @max_by @any @all @contains @sequence_equal @aggregate @first @last @single @element_at

Each has both the direct and piped form and a documentation section in standalonequerycommands.md with a runnable example. All 20 new doctests pass.

Four macros that needed a decision

The piped and direct forms are normally told apart by arity — @take(2) versus @take(df, 2). Four operators broke that, and rather than guess from what an argument looks like, the discriminator is keyword syntax, which is syntactically unambiguous:

• @shuffle takes its generator as rng=: df |> @shuffle(rng=MersenneTwister(42)). A lone positional argument is then always the source.
• @aggregate takes its seed as seed=: df |> @aggregate(f, seed=0), @aggregate(df, f, seed=0).
• @zip has no two-argument piped form with a result selector — it would be indistinguishable from @zip(source, other). df |> @zip(other) |> @map(...) expresses the same thing, so nothing is lost.
• @any, @first, @last, @single mirror the existing @count: no piped form taking a predicate, because a single argument is the source. Write df |> @filter(...) |> @any().

Other notes

Outer joins fill with DataValue, never missing, both before and after the sink. A test asserts the collected rows carry DataValue and that ismissing is false for all of them.

@except_by and @intersect_by deviate from .NET, as flagged in the QueryOperators PR: .NET takes a bare sequence of keys there, while UnionBy takes elements. All three take elements here and apply the selector to both sequences. Documented explicitly in the @union_by section.

@count_by names its key columns the way @summarize does — scalar key becomes key, named tuple key splats — and a test asserts @count_by(_.k) equals @groupby(_.k) |> @map({key=key(_), count=length(_)}).

No new @from clauses. LINQ query-expression syntax has no keywords for these operators, so query_translation.jl is untouched, as is the existing @left_outer_join sugar.

Conflict warning

This touches the export list in src/Query.jl and appends to docs/src/standalonequerycommands.md — the same two files as the pending @summarize PR. Whichever merges second needs a trivial rebase. Happy to rebase this one on top if you merge @summarize first.

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