feat: Add support for coc.nvim

[samhed]

We could previously only get document symbols through Neovim's builtin LSP client. This adds a second request path that uses coc.nvim's "documentSymbols" action.

New API:

• navic.attach_coc(bufnr) to attach manually
• coc = { auto_attach = true } setup option, which attaches to buffers once Coc reports a documentSymbol provider for them.

Note that Coc has no LspAttach equivalent, and providers register asynchronously, so this polls hasProvider on BufEnter/CursorHold until one shows up.

The buffer is marked with navic_client_name = "coc", so is_available(), the lualine component, and the one-client-per-buffer conflict handling all work unchanged.

Closes #53

[samhed]

Tested in Neovim v0.12.4 together with Lazy, Lualine, and Coc-extensions for Lua, C++, Python, and JavaScript.

The relevant configs I used:

lazy.nvim:

{
    dir = "~/devel/nvim-navic",
    dependencies = "neoclide/coc.nvim",
    config = require("setup.navic"),
    event = "VeryLazy",
}

setup.navic:

return function()
    require('nvim-navic').setup({
        coc = {
            auto_attach = true,
        },
    })
end

lualine:

return function()
    require("nvim-navic")
    require("lualine").setup({
      ...
      winbar = {
          lualine_a = {},
          lualine_b = {},
          lualine_c = {},
          lualine_x = {
              { 'filetype', icon_only = true, separator = '' },
              { 'filename', symbols = { unnamed = '' } },
          },
          lualine_y = {
              { "navic", color_correction = nil, navic_opts = nil },
          },
          lualine_z = {},
       ...
    },