Add Parser Support for Auto-Detecting Links in Slice Doc-Comments

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Doxygen will automatically detect and link to elements in doc-comments, even if they weren't explicitly linked to. For example, Doxygen generates 2 links to Locator from this: /// Locator is pretty cool. {@link Locator} exists.

We should consider porting this logic over to the Slice compiler. ie. Augment DocComment::parseFrom to automatically detect links, and call the linkFormatter on them, even if no @link was used.