feat: create belongs-to-many related entities

[elpete]

Closes #84

Review

The two relationship types in this request have different semantics.

• belongsToMany.create: 9/10. The parent, related entity, and single pivot row are unambiguous, so Quick should support this.
• Implicit hasManyThrough.create: 3/10. A parent can have several intermediate entities (and an arbitrarily deep chain), so Quick cannot safely infer which intermediate row should own the new terminal entity.
• Overall recommendation: 7/10, implementing the unambiguous many-to-many case and declining an implicit through-chain write until an API explicitly identifies every required intermediate.

Reasons for

• Makes belongsToMany.create() match developer expectations.
• Returns the newly created related entity.
• Creates the pivot row automatically.
• Supports non-key pivot attributes without requiring a pivot Quick entity.
• Reuses attach(), which now also accepts optional pivot attributes.

Reasons against

• The related insert and pivot insert are separate operations; callers needing atomicity should use their transaction boundary.
• The same pivot attribute struct is applied to every ID when using multi-ID attach.
• A generic hasManyThrough.create would be ambiguous and could associate data with the wrong intermediate entity.

Implementation

var tag = post.tags().create(
    { name : "testing" },
    { context : "created through relationship" }
);

The method creates and returns the loaded related entity, then attaches it. Pivot key values are generated by Quick and cannot be overwritten by the additional pivot attributes.

Test-first evidence

Before implementation, the public post.tags().create(...) call fell through the relationship builder and errored with Method does not exist on QueryBuilder [create].

The integration test now verifies the returned Tag, persisted target row, relationship association, and additional pivot-field value.

Validation

• Focused belongs-to-many spec: 3 passed, 0 failed, 0 errors
• Full suite: 496 passed, 0 failed, 0 errors, 3 skipped
• Formatter completed
• git diff --check passed
• Tested with qb@14.0.0-beta.3