Implementation of the table element

[mortenpi]

The precise implementation of the Table element in CommonMark is currently unclear to me (not sure how the different sub-elements should be organized).

• The Table extension needs to be properly hashed out and documented.
• The conversion from standard library tables also needs to be fixed.
• Remove the TableBody and TableHeader elements altogether, and just interpret the first row as the header.
• Make TableCell a singleton, removing its fields. They could also be turned into some sort of dynamic properties that are determined by traversing the tree.
• Make TableComponent subtype AbstractElement directly and don't use it for Table (to ensure that the internal nodes of a table are not allowed to exists in other contexts).
• Add helper methods like rows, nrows, ncols etc. to dynamically determine the number of columns and rows of a table.

[mortenpi]

A few notes here:

• cmark-gfm tables do not have the TableHeader and TableBody nodes.
• Arguably, TableHeader and TableBody are pretty redundant and make things more complicated. Instead, we could just have TableRows as the children of Table directly, with the first TableRow implicitly interpreted as the header.
• Currently, TableComponent is an AbstractBlock. However, you probably shouldn't use the various table components outside of a Table, so it might make sense to make Table <: AbstractBlock directly and all the other table components <: TableComponent <: AbstractElement.
• If we allow the table to be mutated, then we might end up with different number of TableCells in each row. To make things consistent, we should say that the number of columns of a Table is the longest row and shorter rows are implicitly padded with empty TableCells.

[mortenpi]

A few additional notes:

• CommonMark.jl defines TablePipe <: AbstractInline, and does construct the corresponding Nodes. However, it looks like this is just a temporary helper node for parsing and does not end up in the final AST..?
• It would be helpful to have a rows(::Node) function that, when called on a Table node, returns e.g. header :: TableRow, bodyrows :: Vector{TableRow} that the user can loop over when working with a Table. It would take into account how to properly interpret a bad table, e.g. by merging TableBodys etc.