fix: restore system parser runtime path when vimdoc parser is missing

[mastersilvapt]

This fixes help-buffer Treesitter failures on distro Neovim packages that place parsers in /usr/lib/nvim or /usr/lib64/nvim.

Why

lazy.setup() can rewrite runtimepath, and on some systems that drops /usr/lib/nvim.
Then builtin Treesitter cannot find parser/vimdoc.so, so :help fails with:
Parser could not be created ... language "vimdoc".

What

• Add a small guard in init.lua:
  • If parser/vimdoc.* is not found on current rtp, append /usr/lib/nvim or /usr/lib64/nvim if present.
• Run this guard:
  • before lazy.setup()
  • after lazy.setup() (because rtp can be rewritten there)

This is a no-op on systems where parser is already discoverable.

Validation

• nvim --headless '+h vim.lsp.config' +qall no longer errors
• nvim_get_runtime_file("parser/vimdoc.*", false) returns system parser path