Support Custom Addons Path for Enhanced Editor Flexibility

[IronBlood]

This pull request enables users to specify a custom addons path for 3rd-party libraries, addressing the current limitation where only the VSCode-addons path is used.

Highlights:

• Adds files.updateAddonsPath to let users update the addonsPath if it has not been configured already.
• Validates the provided path using fs.exists after expanding it via util.expandPath.
• Extends the language server's usability beyond VSCode to support editors like Vim/Neovim.

Currently the configuration Lua.addonRepositoryPath has a higher priority, but if it's not set, it won't affect the addon manager in vscode.

Example:

1. Clone the addon repo, let's say the command is execute under /tmp so a folder /tmp/LLS-addons will be created with all the submodules.

   git clone --recurse-submodules https://github.com/LuaLS/LLS-Addons

2. In the client configuration, for example neovim:

   vim.lsp.config('lua_ls', {
     on_init = function(client)
       client.config.settings.Lua = vim.tbl_deep_extend('force', client.config.settings.Lua, {
         -- don't forget the `addons` folder
         -- if it's somewhere under the `$HOME` directory, you can also use `~/path/to/LLS-Addons/addons`
         addonRepositoryPath = '/tmp/LLS-Addons/addons',
         -- other configurations
       })
     end,
     settings = { Lua = {} }
   })

3. To add support of a library (e.g. penlight) in a project, config .luarc.json like this:

   {
       "workspace.library": [
           "${addons}/penlight/module/library",
       ]
   }