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).

scripts/cdoc_grammar.lua

@@ -0,0 +1,87 @@
+--[[!
+LPEG grammar for C doc comments
+]]
+
+--- @class nvim.cdoc.Param
+--- @field kind 'param'
+--- @field name string
+--- @field desc? string
+
+--- @class nvim.cdoc.Return
+--- @field kind 'return'
+--- @field desc string
+
+--- @class nvim.cdoc.Note
+--- @field desc? string
+
+--- @alias nvim.cdoc.grammar.result
+--- | nvim.cdoc.Param
+--- | nvim.cdoc.Return
+--- | nvim.cdoc.Note
+
+--- @class nvim.cdoc.grammar
+--- @field match fun(self, input: string): nvim.cdoc.grammar.result?
+
+local lpeg = vim.lpeg
+local P, R, S = lpeg.P, lpeg.R, lpeg.S
+local Ct, Cg = lpeg.Ct, lpeg.Cg
+
+--- @param x vim.lpeg.Pattern
+local function rep(x)
+  return x ^ 0
+end
+
+--- @param x vim.lpeg.Pattern
+local function rep1(x)
+  return x ^ 1
+end
+
+--- @param x vim.lpeg.Pattern
+local function opt(x)
+  return x ^ -1
+end
+
+local nl = P('\r\n') + P('\n')
+local ws = rep1(S(' \t') + nl)
+
+local any = P(1) -- (consume one character)
+local letter = R('az', 'AZ') + S('_$')
+local ident = letter * rep(letter + R('09'))
+
+local io = P('[') * (P('in') + P('out') + P('inout')) * P(']')
+
+--- @param x string
+local function Pf(x)
+  return opt(ws) * P(x) * opt(ws)
+end
+
+--- @type table<string,vim.lpeg.Pattern>
+local v = setmetatable({}, {
+  __index = function(_, k)
+    return lpeg.V(k)
+  end,
+})
+
+local grammar = P {
+  rep1(P('@') * v.ats),
+
+  ats = v.at_param + v.at_return + v.at_deprecated + v.at_see + v.at_brief + v.at_note + v.at_nodoc,
+
+  at_param = Ct(
+    Cg(P('param'), 'kind') * opt(io) * ws * Cg(ident, 'name') * opt(ws * Cg(rep(any), 'desc'))
+  ),
+
+  at_return = Ct(Cg(P('return'), 'kind') * opt(S('s')) * opt(ws * Cg(rep(any), 'desc'))),
+
+  at_deprecated = Ct(Cg(P('deprecated'), 'kind')),
+
+  at_see = Ct(Cg(P('see'), 'kind') * ws * opt(Pf('#')) * Cg(rep(any), 'desc')),
+
+  at_brief = Ct(Cg(P('brief'), 'kind') * ws * Cg(rep(any), 'desc')),
+
+  at_note = Ct(Cg(P('note'), 'kind') * ws * Cg(rep(any), 'desc')),
+
+  at_nodoc = Ct(Cg(P('nodoc'), 'kind')),
+}
+
+return grammar --[[@as nvim.cdoc.grammar]]