refactor(gen_vimdoc): generate function doc from metadata, not from xml

Problem:

For function definitions to be included in the vimdoc (formatted) and
to be exported as mpack data (unformatted), we had two internal
representations of the same function/API metadata in duplicate;
one is FunctionDoc (which was previously a dict), and the other is
doxygen XML DOM from which vimdoc (functions sections) was generated.

Solution:

We should have a single path and unified data representation
(i.e. FunctionDoc) that contains all the metadata and information about
function APIs, from which both of mpack export and vimdoc are generated.
I.e., vimdocs are no longer generated directly from doxygen XML nodes,
but generated via:

  (XML DOM Nodes) ------------> FunctionDoc ------> mpack (unformatted)
                   Recursive     Internal     |
                   Formatting    Metadata     +---> vimdoc (formatted)

This refactoring eliminates the hacky and ugly use of `fmt_vimhelp` in
`fmt_node_as_vimhelp()` and all other helper functions! This way,
`fmt_node_as_vimhelp()` can simplified as it no longer needs to handle
generating of function docs, which needs to be done only in the topmost
level of recursion.

runtime/lua/vim/_meta/api.lua

@@ -73,6 +73,9 @@ function vim.api.nvim__id_dictionary(dct) end
 function vim.api.nvim__id_float(flt) end
 
 --- @private
+--- NB: if your UI doesn't use hlstate, this will not return hlstate first
+--- time.
+---
 --- @param grid integer
 --- @param row integer
 --- @param col integer