separate `bracketedSpans` plugin from `anchor`

[mesqueeb]

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 feat/bracketed spans #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.