separate `bracketedSpans` plugin from `anchor`

bracketedSpans is sort of a new plugin I want to add (see #10 )
but most of the requested features are already available via links with atributes

common markdown should however not include bracketedSpans (nor links with attributes) but _should_ include just converting links.

- [ ] cut out the logic of the _attributes_ of the `anchor` plugin and save as a **new plugin** called **`bracketedSpans`** this can cover also converting links. When placing the `bracketedSpans` _before_ the `anchor` plugin, it will basically convert your links with additional optional bracketedSpans.
- [ ] make sure the `anchor` plugin still works and leaves the `bracketedSpans` alone if that plugin is not imported
- [ ] make sure the `bracketedSpans` plugin covers all tests added in #10

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please note: before working on this issue, there are some unrelated tests failing on production branch. It's best to first fix all failing tests on production, then pull production into branch of #10 again, then start working on these remaining issues.

Start by checking the unrelated failing tests on the production branch, then review issue #10 and its added tests. Trace the existing anchor plugin's attribute logic and verify how it handles links when bracketedSpans is absent. Done means the new plugin covers the #10 tests, anchor remains compatible, and common Markdown still converts ordinary links.

Written by the indexing model from the issue text.