fix(uri): don't use full path for custom schema

deno lsp uses a custom schema `deno:`

`uri_from_bufnr` returns the full path of the buf file but this doesn't work with custom schema

This fixes the last remaining  bug here <neovim/nvim-lspconfig#2005> `relative dependencies inside virtual text document is not resolved correctly`

Notes:
- maybe `if relative_name:match("deno:")` should become `if is_custom_schema(realtive_name)` where is_custom_schema matches `URI_SCHEME_PATTERN` but is not equal to `file:///`
- this changes fixes all my issues with denols (with some minor modifications that I can pr later)

This is more to start a discussion I thought showing code is easier to discuss then raising an issue

runtime/lua/vim/uri.lua

@@ -83,6 +83,14 @@ local WINDOWS_URI_SCHEME_PATTERN = '^([a-zA-Z]+[a-zA-Z0-9.+-]*):[a-zA-Z]:.*'
 ---@param bufnr number
 ---@return string URI
 local function uri_from_bufnr(bufnr)
+  local relative_name
+  vim.api.nvim_buf_call(bufnr, function()
+    relative_name = vim.api.nvim_exec(":echo @%", true)
+  end)
+  if relative_name:match("deno:") then
+    return relative_name
+  end
+
   local fname = vim.api.nvim_buf_get_name(bufnr)
   local volume_path = fname:match('^([a-zA-Z]:).*')
   local is_windows = volume_path ~= nil