feat(docs): replace lua2dox.lua

Problem:

The documentation flow (`gen_vimdoc.py`) has several issues:
- it's not very versatile
- depends on doxygen
- doesn't work well with Lua code as it requires an awkward filter script to convert it into pseudo-C.
- The intermediate XML files and filters makes it too much like a rube goldberg machine.

Solution:

Re-implement the flow using Lua, LPEG and treesitter.

- `gen_vimdoc.py` is now replaced with `gen_vimdoc.lua` and replicates a portion of the logic.
- `lua2dox.lua` is gone!
- No more XML files.
- Doxygen is now longer used and instead we now use:
  - LPEG for comment parsing (see `scripts/luacats_grammar.lua` and `scripts/cdoc_grammar.lua`).
  - LPEG for C parsing (see `scripts/cdoc_parser.lua`)
  - Lua patterns for Lua parsing (see `scripts/luacats_parser.lua`).
  - Treesitter for Markdown parsing (see `scripts/text_utils.lua`).
- The generated `runtime/doc/*.mpack` files have been removed.
   - `scripts/gen_eval_files.lua` now instead uses `scripts/cdoc_parser.lua` directly.
- Text wrapping is implemented in `scripts/text_utils.lua` and appears to produce more consistent results (the main contributer to the diff of this change).

src/nvim/generators/gen_api_dispatch.lua

@@ -23,9 +23,7 @@ local function_names = {}
 
 local c_grammar = require('generators.c_grammar')
 
-local function startswith(String, Start)
-  return string.sub(String, 1, string.len(Start)) == Start
-end
+local startswith = vim.startswith
 
 local function add_function(fn)
   local public = startswith(fn.name, 'nvim_') or fn.deprecated_since
@@ -112,10 +110,12 @@ for i = 6, #arg do
   local tmp = c_grammar.grammar:match(input:read('*all'))
   for j = 1, #tmp do
     local val = tmp[j]
-    if val.keyset_name then
-      add_keyset(val)
-    else
-      add_function(val)
+    if val[1] ~= 'empty' then
+      if val.keyset_name then
+        add_keyset(val)
+      else
+        add_function(val)
+      end
     end
   end
   input:close()