refactor(api): make typed dicts appear as types in the source code

problem: can we have Serde?
solution: we have Serde at home

This by itself is just a change of notation, that could be quickly
merged to avoid messy merge conflicts, but upcoming changes are planned:

- keysets no longer need to be defined in one single file. `keysets.h` is
  just the initial automatic conversion of the previous `keysets.lua`.
  keysets just used in a single api/{scope}.h can be moved to that file, later on.

- Typed dicts will have more specific types than Object. this will
  enable most of the existing manual typechecking boilerplate to be eliminated.
  We will need some annotation for missing value, i e a boolean will
  need to be represented as a TriState (none/false/true) in some cases.

- Eventually: optional parameters in form of a `Dict opts` final
  parameter will get added in some form to metadata. this will require
  a discussion/desicion about type forward compatibility.

src/nvim/highlight.h

@@ -3,6 +3,7 @@
 
 #include <stdbool.h>
 
+#include "nvim/api/keysets.h"
 #include "nvim/api/private/defs.h"
 #include "nvim/buffer_defs.h"
 #include "nvim/highlight_defs.h"