feat(plugins): add `bracketed_spans`

example:
```md
hello [world]{.c-primary}
```

work on branch: https://github.com/CyCraft/starkdown/pull/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.