fix: stop typographer mangling opening quote of quoted 't' words

[SAY-5]

• goldmark is an extensible library. I will no longer accept PRs that add non-CommonMark features to the core parser. Please implement non-CommonMark features as extensions.
  • If you need addtional goldmark functionalities for implementing non-CommonMark features, please create PR that only adds these functionalities without extensions adding non-CommonMark features.
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The typographer's 'twas/'em/'net contraction shortcut was triggered by any apostrophe-prefixed word starting with t, so quoted words like 'terrifying' or 'them' lost their opening curly quote. The leading 't shortcut now only fires before w/i ('twas, 'tis, 'twixt, ...), while 'twas and friends still work. Closes #347.