Fix Link and Footnote reference definitions in syntax tree

[MihaZupan]

This does not affect HTML rendering at all, it merely exposes more information to the syntax tree:

• Keep all LinkReferenceDefinitions (doesn't overwrite same-named ones)
• Sets the correct lines and spans for them
• Sets the content and source locations for FootnoteLinkReferenceDefinitions

[xoofx]

Thanks for this, looks good, do you think it is complete with enough relevant tests?

[MihaZupan]

Yes.
It is further tested in my project, that currently targets the fork instead of NuGet (it relies on these changes).