[api-extractor] @privateRemarks blocks should be removed from .d.ts rollup files

[octogonz]

Today API Extractor removes the @privateRemarks block from the published documentation; however, it is not removed from the generated .d.ts rollup files. We should improve this, since DocComment.ts says the intended purpose of @privateRemarks is to contain "additional commentary that is not meant for an external audience".

Currently the DtsRollupGenerator class copies the doc comments verbatim into the output. It would be better to use DocComment.emitAsTsdoc() to emit these comments, which would convert them to a normalized form, and also allow us to remove the @privateRemarks blocks.

Before we can do that however, DocComment.emitAsTsdoc() first needs to be improved to support word-wrapping or preserve the original line endings.