Added queryable and enumerable composable expression overloads for condition terminals ordering grouping and flattening

[ycherkes]

Summary

This PR expands composable-expression support across LINQ method and query syntax while preserving the underlying query provider.

• Adds IEnumerable<T> and IQueryable<T> overloads for condition terminals, ordering, grouping, projection, and flattening.
• Introduces the covariant IComposableExpression<TSource, out TResult> consumption contract.
• Allows SelectMany projections returning concrete sequence types such as List<T> and arrays without calling AsEnumerable().
• Keeps ComposableExpression<TSource, TResult> public as the supported base for custom semantic wrappers, expansion, and delegate caching.
• Adds the opt-in Raffinert.Expressions.QuerySyntax package targeting netstandard2.1.
• Adds AsRaffinertQuery() for expanding composition markers throughout C# query syntax without replacing the underlying provider.
• Supports multiple from clauses, joins, group joins, ordering, grouping, continuations, and async enumeration.
• Keeps the core package dependency-free on netstandard2.0.

Implementation details

• LINQ extensions consistently consume IComposableExpression.
• Queryable operators reconstruct typed expressions only when covariance requires adaptation; matching expression types retain their cached instance.
• The query-syntax facade delegates expression, provider, enumeration, and execution to the wrapped query.
• Added an internal expansion entry point shared with the query-syntax package.
• Added provider-independent ToListAsync() and ToArrayAsync() facade methods.
• Updated package metadata, documentation, changelog, solution structure, and CI packaging.

Packages

• Raffinert.Expressions remains on netstandard2.0.
• Raffinert.Expressions.QuerySyntax targets netstandard2.1 and references the core package.
• CI packs and uploads both NuGet and symbol packages.

Testing

• Added unit coverage for all enumerable and queryable extensions.
• Added interface-typed, covariance, List<T>, array, and custom-wrapper coverage.
• Added query-syntax tests covering filtering, projection, multiple from, joins, grouping, ordering, and continuations.
• Added EF Core SQLite integration coverage for expansion, SQL translation, provider preservation, and asynchronous execution.
• All 60 tests pass with zero warnings and errors.