perf(vim.fn): call Lua-implemented vim.fn.xx() directly #39166

Problem:
- Builtin "Vimscript" functions (f_xx) are mostly implemented in C.
  Partly that's because there is some boilerplate required to call out
  to Lua.
- Calls to `vim.fn.foo()` always marshall over the Lua <=> Vimscript
  ("typval") bridge, even if `fn.foo()` is implemented entirely in Lua:
  ```
  Lua => typval => Object => Lua => Object => typval => Lua.
  ```

Solution:
Functions declared in eval.lua with `func_lua` are implemented in
entirely in Lua (`_core/vimfn.lua`).

- `gen_eval.lua` wires `func_lua` entries to `lua_wrapper`, which handles
  the typval conversion for Vimscript callers (slow path).
- `nlua_call()` detects `func_lua` functions and calls the Lua
  implementation directly. This eliminates all conversion overhead for
  Lua callers (fast path).
- Validate at build-time that `func`, `func_float`, and `func_lua` are
  mutually exclusive.
- Migrate `hostname()` as a toy example, to show the idea.

test/functional/vimscript/eval_spec.lua

@@ -215,10 +215,10 @@ describe('listing functions using :function', function()
     command('let A = {-> 1}')
     local num = exec_capture('echo A'):match("function%('<lambda>(%d+)'%)")
     eq(
-      ([[
-   function <lambda>%s(...)
-1  return 1
-   endfunction]]):format(num),
+      t.dedent([[
+           function <lambda>%s(...)
+        1  return 1
+           endfunction]]):format(num),
       exec_capture(('function <lambda>%s'):format(num))
     )
   end)