feat(gen_lsp.lua): validate CLI args #26514

- Improve CLI argument parsing, rejects invalid argument and commands as
  early as possible. Also prints USAGE in the command line.
- No longer allows `--<outfile>`, use `--out <outfile>` instead.
- Print a little bit of verbose messages to better know what's going on
  rather than remaining silent at all times.
- Add type annotation `gen_lsp._opt` to avoid type warnings.

runtime/lua/vim/lsp/_meta/protocol.lua

@@ -1,7 +1,7 @@
 --[[
 This file is autogenerated from scripts/gen_lsp.lua
 Regenerate:
-nvim -l scripts/gen_lsp.lua gen --version 3.18 --runtime/lua/vim/lsp/_meta/protocol.lua
+nvim -l scripts/gen_lsp.lua gen --version 3.18 --out runtime/lua/vim/lsp/_meta/protocol.lua
 --]]
 
 ---@meta