fix: skip _ inside attributes when only the attributes plugin is on

[BetterAndBetterII]

Summary

• With only the attributes plugin enabled, target=_blank no longer auto-closes as italic and appends a leftover underscore.
• Auto-close skips attribute braces whenever attributes is registered, without enabling component fences.

Fixes #362

Test plan

• vitest default-plugins and auto-close tests
• full comark suite 1427 passed, 2 todo

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commit: 70e39bd

[farnabaz]

Thanks 👍