track inner indentation for doxygen commands

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Bug/issue #, if applicable: rdar://107580214

Summary

The initial implementation of Doxygen commands didn't track trailing indentation, i.e. if trailing lines of the description were indented more than the command. This PR adds that tracking, so that commands will be parsed properly if the description text is indented relative to the command name.

The tracking is done similarly to block directive indentation tracking: The first line of the description that trails past the command name sets the indentation for the rest of the description. This is then used to create the offset for parsing the description as a whole.

Dependencies

None

Testing

Use the following markdown:

Method description.

@param thing
    The thing.
 @param otherThing
    The other thing.

Steps:

1. Save the above markdown as test.md.
2. swift run markdown-tool dump-tree --parse-block-directives --experimental-parse-doxygen-commands test.md
3. Compare the output to below:

Document
├─ Paragraph
│  └─ Text "Method description."
├─ DoxygenParameter parameter: thing
│  └─ Paragraph
│     └─ Text "The thing."
└─ DoxygenParameter parameter: otherThing
   └─ Paragraph
      └─ Text "The other thing."

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

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@swift-ci Please test

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@swift-ci Please test

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@swift-ci Please test macOS