feat: support the default switch in the on(...) clause

[ChrisJr404]

Closes #366.

This adds default as a switch in the on(...) clause, so you can write #[builder(on(Vec<_>, default))] to make every matching required member optional and fall back to its Default value. It's the boolean form from your comment on the issue, so a custom default = ... value is still left to #152.

Members that already have a #[builder(default = ...)] or are Option<T> are left untouched, so this doesn't change anything for existing builders. I wired it into the same type-pattern matching that into and overwritable use, added tests to attr_on.rs, and documented the attribute on the on reference page.

[Copilot]

Pull request overview

Adds support for a boolean default switch inside #[builder(on(...))], enabling blanket application of #[builder(default)] to matching required members (without affecting Option<T> members or members that already have a member-level default).

Changes:

• Extend OnConfig parsing to recognize default as a supported on(...) flag.
• Apply on(type_pattern, default) during member config merging to mark matching required members as #[builder(default)].
• Add integration tests and update the website reference docs with the new default usage and limitations.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
website/src/reference/builder/top-level/on.md	Documents default as a supported on(...) flag and adds an example + clarifies boolean-only limitation.
bon/tests/integration/builder/attr_on.rs	Adds coverage for blanket-default matching (path + generic) and interaction with into and member-level defaults.
bon-macros/src/builder/builder_gen/top_level_config/on.rs	Extends on(...) meta parsing to accept the default flag.
bon-macros/src/builder/builder_gen/member/named.rs	Implements merging logic to apply on(..., default) to matching required members while leaving optional/defaulted members untouched.

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[Veetaha]

Thanks for the PR, I'll release it soon