Add Between/NotBetween expression helpers

[sonnemusk]

Summary

Adds first-class BETWEEN / NOT BETWEEN support to squirrel's expression helpers, addressing #340.

Today the common workaround is:

.Where(sq.Expr("age BETWEEN ? AND ?", 18, 65))

This PR makes that consistent with Eq, Lt, Like, and friends:

.Where(sq.Between{"age": []int{18, 65}})
// → age BETWEEN ? AND ?

.Where(sq.NotBetween{"id": []int{1, 5}})
// → id NOT BETWEEN ? AND ?

// Short alias requested in #340
.Where(sq.Bt{"score": []interface{}{0, 100}})
// → score BETWEEN ? AND ?

API design

• Between / NotBetween: map[string]interface{}, same multi-column AND style as Eq / Lt.
• Values must be a two-element list or array (lower, upper). Other lengths, non-list values, and nil return an error.
• Bt: type alias for Between (as suggested in the issue).
• Empty Between{} evaluates to (1=1), matching empty Eq{}.
• Bounds are bound as placeholders (?), so they work with PlaceholderFormat (dollar, colon, etc.).

A previous attempt in #180 used an unexported-field struct and was never usable; this follows the established map-based pattern instead.

Tests

Unit coverage in expr_test.go for:

• basic Between / NotBetween / Bt
• []T, []interface{}, and [2]T bounds
• multi-key stable ordering
• empty map
• nil / non-list / wrong-length errors
• use inside Select(...).Where(...)

go test ./...

Related

Closes #340